### Problem
ubuntu-latest was updated to 22.04 which removes clang++-10 we used for
coverage stats and creates a pre-compiled binary that requires a glibc
upgrade https://github.com/Roblox/luau/issues/773
### Solution
Pin to ubuntu-20.04 using multi-value matrix configurations.
In coverage configuration, we use clang++-10 once again.
We don't need to run any cachegrind benchmarks in benchmark-dev, since
benchmark uses our new callgrind setup instead.
Also removes prototyping filters that we no longer need from all builds.
Changed the GHA workflows to:
- Not run `build` and `release` workflows for PRs that only affect `prototyping/`
- Run `prototyping` workflow when PRs affect `Analysis/**`, `Ast/**`, or the `luau-ast` source files
