feat: add GitHub Enterprise Server (GHES) support (#36)

This adds support for this action to be used in GitHub Enterprise
Server.

It sends request to the base url extracted from
[GITHUB_API_URL](https://docs.github.com/en/enterprise-server@3.10/actions/learn-github-actions/variables#default-environment-variables).

---------

Co-authored-by: Gregor Martynus <39992+gr2m@users.noreply.github.com>

lib/post.js

@@ -11,7 +11,7 @@ export async function post(core, request) {
   const token = core.getState("token");
 
   if (!token) return;
-  
+
   await request("DELETE /installation/token", {
     headers: {
       authorization: `token ${token}`,