Prevent duplicate release drafts

[itsrainy]

Our github actions currently have a race condition that can cause two separate draft releases to get created if two build jobs finish at the same time. This happened for the 0.4.4 release:

This resulted in some artifacts not actually getting uploaded to the correct release.

I think we can use Github's concurrency mechanism at the job level to ensure that a draft release already exists before any of the build jobs get to this step.