Add support for and rotate between mirrors

[weskoerber]

The README for mlugg/setup-zig says this:

If you want to use one specific mirror, you can set it using the mirror option:

  - uses: mlugg/setup-zig@v1
    with:
      mirror: 'https://pkg.machengine.org/zig'

Please don't do this unnecessarily; it's not nice to hammer one mirror. This mirror is not permitted to be https://ziglang.org/download to avoid the official website being hit with large amounts of requests. 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.

Zvm should play by these rules as well, since right now the only mirror it supports is the official site which is explicitly called out to not do this.