post_version.py: fix branch name construction for release candidates

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2870
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4929>
1 file changed