GHA improvements

[penelopeysm]

• Fix paths for caching Rust dependencies (CLI generator script #27 broke it)
  • Additionally, make the key depend on Cargo.lock contents (but still using rust_backend as a restore key). Previously we'd been doing it slightly carelessly by having a static key — this means that the cache would never be updated if the dependencies changed
• Adds caching for pnpm dependencies (minor speedup)
• Test packages in parallel rather than sequentially — this both speeds things up and also makes it easier to see if something is failing in multiple parts of the codebase
• Only deploy the website to GitHub Pages if tests pass. This was done by removing the separate web.yml and looping in the deployment step into the other action.
• This isn't part of the code, but I installed the pre-commit-ci-lite GitHub app on this repo so that the corresponding action would work.

[penelopeysm]

Runs well on this PR, so will merge.