Add lookup for Mach nominated versions

Author: mlugg

README.md

@@ -19,7 +19,8 @@ This will automatically download Zig and install it to `PATH`.
 
 You can use `version` to set a Zig version to download. This may be a release (`0.13.0`), a specific nightly
 build (`0.14.0-dev.2+0884a4341`), the string `master` for the latest nightly build, or the string `latest`
-for the latest full release. The default is `latest`.
+for the latest full release. It can also refer to a [Mach nominated version][mach-nominated], such as
+`2024.5.0-mach`. The default is `latest`.
 
 ```yaml
   - uses: mlugg/setup-zig@v1
@@ -44,6 +45,8 @@ be https://ziglang.org/download to avoid the oggicial websote being hit with lar
 If you've experienced issues with a default mirror, please open an issue, and I will communicate with the
 mirror's owner or remove it from the list.
 
+[mach-nominated]: https://machengine.org/about/nominated-zig/
+
 ## Details
 
 This action attempts to download the requested Zig tarball from a set of mirrors, in a random order. As

common.js

@@ -5,6 +5,7 @@ const github = require('@actions/github');
 const exec = require('@actions/exec');
 
 const VERSIONS_JSON = 'https://ziglang.org/download/index.json';
+const MACH_VERSIONS_JSON = 'https://pkg.machengine.org/zig/index.json';
 const CACHE_PREFIX = "setup-zig-global-cache-";
 
 let _cached_version = null;
@@ -49,6 +50,13 @@ async function getVersion() {
       }
     }
     _cached_version = latest;
+  } else if (raw.includes("mach")) {
+    const resp = await fetch(MACH_VERSIONS_JSON);
+    const versions = await resp.json();
+    if (!(raw in versions)) {
+      throw new Error(`Mach nominated version '${raw}' not found`);
+    }
+    _cached_version = versions[raw].version;
   } else {
     _cached_version = raw;
   }