next.js

b8d268d0 - [ci] Download fnm directly from GitHub releases and validate checksum (#94579)

[ci] Download fnm directly from GitHub releases and validate checksum (#94579) I determined that [`nvm` wasn't sufficient for our use-case since we need to support windows](https://github.com/vercel/next.js/pull/94175#issuecomment-4654622674), so this doubles down on `fnm`. This reduces risks of supply chain attacks with `fnm`. This also cleans up some old stuff that was left over from self-hosted runners and/or that was there for debugging, if it isn't useful anymore.