Add sysroot support to build.rs scripts.

[sayrer]

If there's a sysroot, it needs to appear before any other flags. See: #659.

[illicitonion]

Thanks!

[hlopko]

Could you add a unit test? Hopefully #608 is a good starting example?

[sayrer]

Could you add a unit test? Hopefully #608 is a good starting example?

I'm not really certain how I would add a unit test for this, at least in that style. This patch doesn't change the analysis. edit: Perhaps I'm meant to check https://docs.bazel.build/versions/master/skylark/lib/Action.html#env for all of this stuff? FWIW, I think it would be easy to add a test for this if anything else was tested in this way. I just don't know that much about how the skylib test library works, or whether cargo_build_script.bzl is suitably factored to be tested this way.

[hlopko]

Yeah you're right, using a cc toolchain with a sysroot in a unittest is hard. Ignore my comment. @illicitonion can you merge this PR (since you approved and didn't merge I don't want to step in)?

[illicitonion]

I thought I pressed the merge button when I approved! ... Weird.

[illicitonion]

Oh, looks like I don't have permission to merge main - @sayrer could you rebase/merge main and I'll merge to main? Thanks!

[sayrer]

Huh, now I'm getting Buildkite complaints about files that aren't in my patch. Will take a look later on today.

[UebelAndre]

Seems like buildifier was fixed 😄

[sayrer]

Put up #675. It seems like I just got unlucky here.

[hlopko]

And we'll need a merge again :/

[hlopko]

Thank you!