Set UserAgent to 'actions-runner-controller' for all Http Client. (#1140)

I can't find any requests made by user agent `actions-runner-controller` in GitHub.com's telemetry in the past 7 days.

Turns out we only set user agent `actions-runner-controller` if we are configured to use BasicAuth which is not the case for most customers I think.

I update the code a little bit to make sure it always set `actions-runner-controller` as UserAgent for the GitHub HttpClient in ARC.

A further step would be somehow baking the ARC release version into the UserAgent as well.

github/github_test.go

@@ -152,3 +152,10 @@ func TestCleanup(t *testing.T) {
 		t.Errorf("expired token still exists")
 	}
 }
+
+func TestUserAgent(t *testing.T) {
+	client := newTestClient()
+	if client.UserAgent != "actions-runner-controller" {
+		t.Errorf("UserAgent should be set to actions-runner-controller")
+	}
+}