Fix TestGnuPGHome_Validate

[stormentt]

On my stock Ubuntu 22.04 machine with Go 1.21 one of the tests fails:

--- FAIL: TestGnuPGHome_Validate (0.00s)
    --- FAIL: TestGnuPGHome_Validate/wrong_permissions (0.00s)
        keysource_test.go:134: 
                Error Trace:    /home/xmo/pro/sops/pgp/keysource_test.go:134
                Error:          An error is expected but got nil.
                Test:           TestGnuPGHome_Validate/wrong_permissions

The test assumes that t.TempDir() returns a directory with mode 0755.

Taking a look at testing.go:1221 shows that this uses os.MkdirTemp(), which calls Mkdir(name, 0700), causing the test to fail.

To fix this I modified the test to explicitly set the mode on the tempdir to 0755.

[felixfontein]

@stormentt thanks a lot for fixing this!