- use `path` instead of `paths`
- don't mark rust-analyzer as an optional tool
- print the cargo command that's run in the proc-macro-test build script
this originally was part of a change to fix `test --stage 0 rust-analyzer`,
but I'm going to leave that for a separate PR so it's easier to review.
Building it in-place fails in rust CI because the source directory
is read-only. This changes `proc-macro-test`'s build script to first
copy `imp` under `OUT_DIR` (which is read-write).
It also prints stdout/stderr for the nested cargo invocation, should
it fail. (I've seen failures in rust CI that I couldn't explain, and
when they take 25 minutes to reproduce, you want to have that info)
