use hardcoded community mirror list when ziglang.org is inaccessible

main.js

@@ -51,14 +51,31 @@ async function downloadTarball(tarball_filename) {
     return await downloadFromMirror(preferred_mirror, tarball_filename);
   }
 
-  // Fetch the list of mirrors from ziglang.org. Caching the mirror list is awkward in this context,
-  // so if the list is inaccessible, we just fetch from ziglang.org as a fallback.
-  let mirrors = [];
+  let mirrors;
   try {
+    // Fetch an up-to-date list of available mirrors from ziglang.org.
     const mirrors_response = await fetch(MIRRORS_URL);
     mirrors = (await mirrors_response.text()).split('\n').filter((url) => url.length != 0);
   } catch {
-    // For some reason the mirrors are inaccessible. That's okay; allow ourselves to fall back to ziglang.org below.
+    // ziglang.org itself is inaccessible. As a fallback (so that ziglang.org outages don't break
+    // this Action), use a hardcoded mirror list. This was manually fetched from ziglang.org, and I
+    // expect that at least some of these mirrors will continue working. It only needs to be updated
+    // if the upstream list changes *significantly*.
+    mirrors = [
+      "https://pkg.machengine.org/zig",
+      "https://zigmirror.hryx.net/zig",
+      "https://zig.linus.dev/zig",
+      "https://zig.squirl.dev",
+      "https://zig.florent.dev",
+      "https://zig.mirror.mschae23.de/zig",
+      "https://zigmirror.meox.dev",
+      "https://ziglang.freetls.fastly.net",
+      "https://zig.tilok.dev",
+      "https://zig-mirror.tsimnet.eu/zig",
+      "https://zig.karearl.com/zig",
+      "https://pkg.earth/zig",
+      "https://fs.liujiacai.net/zigbuilds",
+    ];
   }
 
   core.info(`Available mirrors: ${mirrors.join(", ")}`);