CI: use mlugg/setup-zig

[mlugg]

This doesn't need to be merged if you'd prefer to stick with the current logic; I largely just wanted to see if it works properly. Nonetheless, I think this represents an improvement to the CI scripts.

This PR begins using the https://github.com/mlugg/setup-zig action to install Zig in the CI workflow. This has the following advantages:

• No more target-specific scripts; setting up Zig is 4 simple lines.
• The tarball's minisig is verified, so this will flag up issues if the Mach mirror ever starts doing anything weird.
• The Zig version is specified in Mach lingo, i.e. 2024.5.0-mach rather than zig-0.13.0-dev.351+64ef45eb0.
  • This could alternatively say mach-latest, but I figured this makes more sense for handling the upgrade process when new versions are nominated.
• The global Zig cache is automatically preserved across runs, avoiding redundant work. In particular, this will avoid doing redundant work rebuilding C dependencies on every commit.

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