ci: downgrade libc to 0.2.171 in CI for hurd

[SteveLauC]

What does this PR do

1. In our CI, pin libc to 0.2.171 on hurd to work around this issue: 0.2.172 is broken on hurd rust-lang/libc#4421
2. Bump CI runner images, Ubuntu-20.04 is deprecated.
3. Ignore test test_prctl::test_set_vma_anon_name under QEMU due to ptctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, addr, len, invalid_name) returns EFAULT rather than EINVAL under QEMU #2418
4. Allow clippy::unwrap_or_default as IMHO it is overly opinionated

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[Berrysoft]

Please don't do that, because cygwin needs 0.2.172

EDIT: how about just to pin libc to 0.2.171 on hurd?

[SteveLauC]

how about just to pin libc to 0.2.171 on hurd?

Sounds good, I will do it

[asomers]

Instead of pinning libc in Cargo.toml, I think you should do it just in the CI configuration . On the hurd target, do an extra step of cargo update -p libc --precise 0.2.171. That way we won't be limiting the libc version that our consumers can use.