Avoid MacOS keychain dialog in integration tests (#32813)

Mac's git installation ships with a system wide config that configures
the credential helper `osxkeychain`, which will prompt the user with a
dialog.

```
$ git config list --system 
credential.helper=osxkeychain
```
By setting the environment variable
[`GIT_CONFIG_NOSYSTEM=true`](https://git-scm.com/docs/git-config#ENVIRONMENT),
Git will not load the system wide config, preventing the dialog from
populating.

Closes #26717

tests/integration/integration_test.go

@@ -118,6 +118,11 @@ func TestMain(m *testing.M) {
 	os.Unsetenv("GIT_COMMITTER_EMAIL")
 	os.Unsetenv("GIT_COMMITTER_DATE")
 
+	// Avoid loading the default system config. On MacOS, this config
+	// sets the osxkeychain credential helper, which will cause tests
+	// to freeze with a dialog.
+	os.Setenv("GIT_CONFIG_NOSYSTEM", "true")
+
 	err := unittest.InitFixtures(
 		unittest.FixturesOptions{
 			Dir: filepath.Join(filepath.Dir(setting.AppPath), "models/fixtures/"),