[ci skip] ci: allow prs from nightly and beta to go into release

[derklaro]

Motivation

Currently only prs from "nightly" can be merged into the "release" branch without the prbranch worklow failing. This is incorrect as prs from beta into release should be permitted as well.

Modification

Allow prs from branch "beta" to target "release" by excluding beta and nightly from the check.

Result

PRs from beta can now be created to target release without ci failures.