[CI/Build] Build on Ubuntu 20.04 instead of 22.04

[tlrmchlsmth]

Downgrade the Docker images so we build on Ubuntu 20.04 instead of 22.04

The main effect is to lower the version of GLIBC required for vLLM. This should reduce issues like #6462 happening in the future (but not completely prevent them).

In the future we might want to consider switching to Rocky Linux 8:

• Rocky Linux is binary compatible with RHEL
• manylinux_2_28 is based on AlmaLinux 8. See https://github.com/pypa/manylinux. (AlmaLinux is also supposed to be binary compatible with RHEL)
• NVIDIA has docker images for it https://hub.docker.com/r/nvidia/cuda/
  That could put us on a path towards a more official manylinux support

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[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only trigger fastcheck CI to run, which consists only a small and essential subset of tests to quickly catch errors with the flexibility to run extra individual tests on top (you can do this by unblocking test steps in the Buildkite run).

Full CI run is still required to merge this PR so once the PR is ready to go, please make sure to run it. If you need all test signals in between PR commits, you can trigger full CI as well.

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[simon-mo]

LGTM. If the tests for other hardwares fail, feel free just to focus the change on Dockerfile

[tlrmchlsmth]

/ready

[tlrmchlsmth]

Just updated the docker to use the python 3.8 wheel file for flashinfer -- we may want to upgrade to use a newer version of python but this was easier for now

[rkooo567]

good change!

[tlrmchlsmth]

@youkaichao and I are trying to see if we can remove the glibc dependency in 0.5.2 completely. It wasn't there in 0.5.1 at all.

Meanwhile, not sure why the build is red for this PR but I've been running into some python and pip versioning issues