Cache builds better

[NotNite]

Beyond #1 and #2, the biggest issue with CI speed is that we clone the repository and install dependencies every run without caching. This takes up a lot of time, especially bulk updates in one PR. Some proposals:

• Clone each unique repository once in the runner, and just check out multiple commits when doing multiple builds.
  • There could be some funny business with a previous run being able to mutably write to the repository, or giving it .git access. We'd probably want to copy the repository into a separate temp folder, to be mounted and then deleted.
  • Need to make sure that cloning the repository from the runner doesn't have any security problems.
• Fetch dependencies ahead of time and cache the pnpm store between runs.
  • Lets us build extensions fully offline, which would be a nice extra layer of security.
  • Could be dangerous if we let build scripts run by accident.
  • Not sure how this'll interact with Allow custom package managers #2 if we start using Corepack.
  • Sharing the cache between all extensions sounds dangerous and wasteful. Maybe we could make a read-only cache of the common dependencies included in the extension template, and any other added dependencies have to be pulled every run.