Fix up CI so that CRAN release can be pushed as a PR

[MichaelChirico]

In #6829 @TysonStanley and I discussed pushing the as-submitted CRAN release to GitHub to allow other Committers to merge it once the update is accepted by CRAN.

That would help minimize the time between a CRAN acceptance and an update here, without requiring the CRAN maintainer to operate with 24/7 availability.

However, we do have this note in the CRAN release today:

data.table/.dev/CRAN_Release.cmd

Line 601 in 299d44d

# DO NOT even use a PR. Because PRs build binaries and we don't want any binary versions of even release numbers available from anywhere other than CRAN.

IINM, we should be able to update the CI configs so that certain branches/rules don't trigger the generation of binaries, etc. & thus avoid the concern mentioned here.

E.g., the rule could be "push to a branch specifically named cran-release", and our configs actively exclude that specific name.