Actually implement the mirror restriction

Forgot to actually do this, oopsie

Author: mlugg

main.js

@@ -39,6 +39,9 @@ async function downloadFromMirror(mirror, tarball_name, tarball_ext) {
 
 async function downloadTarball(tarball_name, tarball_ext) {
   const preferred_mirror = core.getInput('mirror');
+  if (preferred_mirror.includes("://ziglang.org/")) {
+    throw new Error("'https://ziglang.org' cannot be used as mirror override; for more information see README.md");
+  }
   if (preferred_mirror) {
     core.info(`Using mirror: ${preferred_mirror}`);
     return await downloadFromMirror(preferred_mirror, tarball_name, tarball_ext);