Add workingDirectory option

Let user override $GITHUB_WORKSPACE as default working directory

Defaults to undefined, the original behaviour is maintained

__test__/git-auth-helper.test.ts

@@ -824,7 +824,8 @@ async function setup(testName: string): Promise<void> {
     sshUser: '',
     workflowOrganizationId: 123456,
     setSafeDirectory: true,
-    githubServerUrl: githubServerUrl
+    githubServerUrl: githubServerUrl,
+    workingDirectory: undefined
   }
 }