We need ubuntu-20.04 for coverage analysis (clang after 10 doesn't seem
to properly interact with gcov version used for codecov) and for
releases (to produce binaries targeting earlier glibc).
However, we still should be verifying that Luau builds on latest,
because newer toolchains have stricter standard library headers and/or
warnings; without this we're at risk of constantly regressing the build
for packaging or external applications.
Note that release.yml can probably just be deleted but for now we simply
adjust it.
### 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.
