Support GitHub Enterprise Instances

[maxmoehl]

Currently this action does not supply a API URL to the GitHub client:

merge-gatekeeper/internal/github/github.go

Lines 34 to 38 in 12e1af4

	ghc: github.NewClient(oauth2.NewClient(ctx, oauth2.StaticTokenSource(
	&oauth2.Token{
	AccessToken: token,
	},
	))),

According to the documentation this causes the client to default to the GitHub.com API. Within Actions the correct API URL for the instance is available as GITHUB_API_URL.

I'd be happy to provide a PR to fix this!