PHD: build crucible-downstairs automatically if Crucible dep is patched

[hawkw]

Currently (as of #778) we force users who have patched the Crucible dep to bring their own downstairs binary in order to build tests. It would be cool if we could detect patched deps and just go build it.

Off-the-wall idea here, but: is it possible to get cargo xtask phd to notice that the Crucible path has been patched and to build a downstairs binary from that path? We could then try to pipe it into a (separate) environment var and have --crucible-downstairs-commit auto try to pick that up: the git rev isn't valid, but I have a proposed path to a Crucible downstairs on the local machine, so let me try that instead.

Originally posted by @gjcolombo in #778 (review)

I wonder if there's some way to (ab)use cargo binary dependencies to accomplish this nicely...