Add and use Mill wrapper script (backport #4119) (#4124)

* Add and use Mill wrapper script (#4119)

This saves developers from having to install a dependency. It also
allows us to drop an old Github Actions dependency that hasn't been
updated in 2 years. This is the standard practice for projects using
Mill on Github.

(cherry picked from commit 0f2efb52

)

# Conflicts:
#	.github/workflows/test.yml
#	CONTRIBUTING.md
#	benchmark/src/main/scala/chisel3/benchmark/Benchmark.scala

* Resolve backport conflicts

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Co-authored-by: Jack Koenig <koenig@sifive.com>