Run limited release CI job on release PR

We've had several instances where the release CI job fails to run as expected (or, in our most recent instance, at all) and we didn't notice until trying to perform the release, which meant that we've had to release a point release. This is bothersome for the core team member doing the release, and it necessitates a bit of scrambling at the last minute.

I propose we solve this problem by updating the release CI job to run a slightly restricted version if the branch is release-next. We would adjust the packagecloud.io script to have a --dry-run mode that doesn't actually upload items but prints a message and then invoke it that way when we're running on the release-next branch. Alternately, we can create a simple -pre0 tag instead and similarly use the --dry-run mode in that case.

Opinions welcome from @git-lfs/core or anyone else.