GitHub Enterprise support (#658)

* Add option to specify custom GitHub instance

* Use correct GHE API endpoint URLs

Co-authored-by: Markus Wolf <[email protected]>

* Extract slug from GitHub Enterprise URLs

Co-authored-by: Markus Wolf <[email protected]>

* Use GITHUB_TOKEN for clone authenticate if provided

This change will allow use authentication for cloning actions
from private repositories or github enterprise instances.

Co-Authored-By: Markus Wolf <[email protected]>

* Add section about using act on GitHub Enterprise to README

Co-authored-by: Markus Wolf <[email protected]>

* Set GitHubInstance in runnerConfig in runner_test

Co-authored-by: Markus Wolf <[email protected]>

Co-authored-by: hackercat <[email protected]>
Co-authored-by: Markus Wolf <[email protected]>

pkg/runner/runner_test.go

@@ -56,6 +56,7 @@ func runTestJobFile(ctx context.Context, t *testing.T, tjfi TestJobFileInfo, sec
 			ReuseContainers:       false,
 			ContainerArchitecture: tjfi.containerArchitecture,
 			Secrets:               secrets,
+			GitHubInstance:        "github.com",
 		}
 
 		runner, err := New(runnerConfig)