`prerelease: false` was hardcoded, so dispatching create-release with
`1.84.0rc1`, `1.84.0.dev42`, or legacy `v1.83.13-nightly` would publish
them as stable releases on the GitHub Releases page. Derive the flag
from the tag instead.
The detector matches `rc`, `.dev`, `nightly`, `alpha`, `beta`. PEP 440
post-releases (`1.84.0.post1`) and legacy `-stable[.patch.N]` are
stable maintenance releases per PEP 440, so they intentionally do not
match.
3a5980804c
