The great cache update and byond mirror fallback

[Drulikar]

About the pull request

This PR updates most caching in CI:

• Depreciated cache@v3 usage is updated to cache@v4
• Byond downloads now test the archive downloaded, and support a mirror to download builds from
• Restore keys for all cache usage changed
• Purge key secret is nuked (not useful when you can just delete the cache)
• Byond cache keys now include the build version (mostly ports https://github.com/tgstation/tgstation/pull/89180)
• Bootstrap is now cached (mostly ports https://github.com/tgstation/tgstation/pull/78307)
• Node is cached differently and Yarn is now cached with Node (mostly ports https://github.com/tgstation/tgstation/pull/89363 and https://github.com/tgstation/tgstation/pull/89494)

Explain why it's good for the game

More reliable and flexible CI.

Testing Photographs and Procedure

See checks.

Changelog

:cl: Drathek code: CI now caches/installs Byond, spacemandmm, rust, bootstrap, node, and yarn differently for Github actions /:cl: