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.

changed milestone to %Arti 0.1.0 release: Okay for experimental embedding

requested review from @eta

assigned to @nickm

approved this merge request

enabled an automatic merge when the pipeline for ef5dc3cb succeeds

merged

mentioned in commit 6e94d5fd