with_coverage: Allow toolchain selection.

There's some kind of bug in nightly that (for me) keeps grcov from
generating correct results. Instead, it misses some crates entirely.

No trouble, though: I can just use an older nightly until they get
this issue fixed!

This patch makes using a different version of nightly possible with
a new RUST_COVERAGE_TOOLCHAIN environment variable.
5 jobs for coverage_toolchain in 27 minutes and 27 seconds (queued for 1 second)