Fix missing extension in temporary tarball path

For some reason, this was breaking archive extraction on Windows only on
Forgejo Actions (not GitHub Actions).

common.js

@@ -248,15 +248,10 @@ function parseZigString(raw) {
   return result;
 }
 
-async function getTarballCachePath() {
-  return path.join(process.env['RUNNER_TEMP'], await getTarballName());
-}
-
 module.exports = {
   getVersion,
   getTarballName,
   getTarballExt,
   getCachePrefix,
   getZigCachePath,
-  getTarballCachePath,
 };

main.js

@@ -85,14 +85,15 @@ async function downloadTarball(tarball_filename) {
 
 async function retrieveTarball(tarball_name, tarball_ext) {
   const cache_key = `setup-zig-tarball-${tarball_name}`;
-  const tarball_cache_path = await common.getTarballCachePath();
+  const tarball_basename = `${tarball_name}${tarball_ext}`;
+  const tarball_cache_path = path.join(process.env['RUNNER_TEMP'], tarball_basename);
 
   if (await cache.restoreCache([tarball_cache_path], cache_key)) {
     return tarball_cache_path;
   }
 
   core.info(`Cache miss. Fetching Zig ${await common.getVersion()}`);
-  const downloaded_path = await downloadTarball(`${tarball_name}${tarball_ext}`);
+  const downloaded_path = await downloadTarball(tarball_basename);
   await fs.copyFile(downloaded_path, tarball_cache_path)
   await cache.saveCache([tarball_cache_path], cache_key);
   return tarball_cache_path;