Release branch calculation does not support prereleases

[nrb]

/kind bug
/kind cleanup

The commit that introduced gorelease building includes the following line for setting the RELEASE_BRANCH variable:

echo "RELEASE_BRANCH=release-$(echo ${GITHUB_REF_NAME} | grep -Eo '[0-9]\.[0-9]+')" >> $GITHUB_ENV

This works for new minor and patch releases, but for prerelease versions, the value should be set to main.

[nrb]

/triage accepted

[nrb]

/area release