.github: Attempt to un-break Clang 9 action

GitHub removed Clang 9 from the 20.04 image[1], breaking this build.
Thus, manually add the specific versioned packages we need for the
Ubuntu jobs to ensure they're installed. Note that we don't do the same
for macOS, as Homebrew does not allow multiple llvm@N to co-exist,
giving an error if you attempt to install a second one. In practice we
don't actually use the compiler field here for anything other than the
build name, it's only the cross-bindir that matters, so when it
eventually moves to 12 the name will get confusing but the job will
still work.

MFC after: immediately

[1] https://github.com/actions/virtual-environments/commit/15a610677be406d250c1f6732b03c8b87e693a0a