fix(Makefile): set locale to make tests pass

[jonasbadstuebner]

Matching on error messages without specifying locale failed for my German setup

Error Message

--- FAIL: TestGnuPGHome_Import (0.02s)                                                                                                                                                                                                                                                                                        
    keysource_test.go:71:                                                                                                                                                                                                                                                                                                     
                Error Trace:    /home/jbadstuebner/Dokumente/git/github.com/jonasbadstuebner/sops/main/pgp/keysource_test.go:71                                                                                                                                                                                               
                Error:          Error "failed to import armored key data into GnuPG keyring (exit status 2): gpg: Keine gültigen OpenPGP-Daten gefunden.\ngpg: Anzahl insgesamt bearbeiteter Schlüssel: 0" does not contain "(exit status 2): gpg: no valid OpenPGP data found.\ngpg: Total number processed: 0"              
                Test:           TestGnuPGHome_Import                                                                                                                                                                                                                                                                          
--- FAIL: TestMasterKey_encryptWithGnuPG (0.04s)                                                                                                                                                                                                                                                                              
    --- FAIL: TestMasterKey_encryptWithGnuPG/invalid_fingerprint_error (0.00s)                                                                                                                                                                                                                                                
        keysource_test.go:303:                                                                                                                                                                                                                                                                                                
                Error Trace:    /home/jbadstuebner/Dokumente/git/github.com/jonasbadstuebner/sops/main/pgp/keysource_test.go:303                                                                                                                                                                                              
                Error:          Error "failed to encrypt sops data key with pgp: gpg: 'invalid' ist keine gültige lange Schlüssel-ID" does not contain "failed to encrypt sops data key with pgp: gpg: 'invalid' is not a valid long keyID"                                                                                   
                Test:           TestMasterKey_encryptWithGnuPG/invalid_fingerprint_error                                                                                                                                                                                                                                      
--- FAIL: TestMasterKey_decryptWithGnuPG (0.04s)                                                                                                                                                                                                                                                                              
    --- FAIL: TestMasterKey_decryptWithGnuPG/invalid_data_error (0.00s)                                                                                                                                                                                                                                                       
        keysource_test.go:507:                                                                                                                                 
                Error Trace:    /home/jbadstuebner/Dokumente/git/github.com/jonasbadstuebner/sops/main/pgp/keysource_test.go:507     
                Error:          Error "failed to decrypt sops data key with pgp: gpg: Keine gültigen OpenPGP-Daten gefunden.\ngpg: decrypt_message failed: Unbekannter Systemfehler" does not contain "gpg: no valid OpenPGP data found"
                Test:           TestMasterKey_decryptWithGnuPG/invalid_data_error                                                    
FAIL

[jonasbadstuebner]

I'm talking about running make test locally

[felixfontein]

Looks reasonable to me.

[jonasbadstuebner]

Would love to see this merged for my peace of mind.

[sabre1041]

LGTM

[felixfontein]

@jonasbadstuebner thanks for your contribution!
@sabre1041 thanks for reviewing!