Support pr channels

[LilithHafner]

It would be lovely to have julia +pr54152 download and run the binaries built from the pull request # 54152 (if binaries are available). If the build failed or the PR is old enough that the binaries have been discarded or my system is not compatible with any of the CI systems then I wouldn't expect this to be possible, but in most cases I care about none of those are the case. It would make interactive testing of a PR much nicer to e.g. verify that purported performance improvements are real.

I recall hearing on slack that with the new nightly support this shouldn't be too hard, but I can't find that conversation now.