haskell-ci.yml: try to fix node20 glibc mismatch

[juhp]

ubuntu:20.04 is focal so try that for buildpack-deps

This is to fix: https://github.com/glguy/config-value/actions/runs/10949184645/job/30401853700

Run actions/checkout@v3
/usr/bin/docker exec  8f6e793bdab35a403b58198eaf1369624e6f8fc706ca491d621d31d67f16767b sh -c "cat /etc/*release | grep ^ID"
/__e/node20/bin/node: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /__e/node20/bin/node)

Though I really don't know if it is sufficient: I see other haskell-ci projects using buildpack-deps:jammy 🤷🏻

[juhp]

I personally find haskell-ci far too complicated and just use the simplest thing I could come up with:
eg https://github.com/juhp/select-rpms/blob/main/.github/workflows/build.yml

[juhp]

just fixed up the commit message (jammy -> focal): don't even know why haskell-ci is using the code-names but anyway.