Build version from default branch with lower priority

[mmuetzel]

Currently, the version from the default branch is built first. That can lead to the impression that it is the 64-bit version that testers should use (see, e.g., https://savannah.gnu.org/bugs/index.php?64024). But it is "expected" to break more often than the versions from the release branch.

It might be better to build the default-w64 version last. Imho, it would also be the smallest loss if that version wouldn't yet have finished building when the CI here starts.

It looks like the builds are done in alphabetical order of their builder names.

IIUC, it could be possible to add a nextBuild function to do that:
https://docs.buildbot.net/latest/manual/customization.html#build-priority-functions
The first example might actually work for us here (build the rules using the release branch before anything else):

def nextBuild(bldr, requests):
    for r in requests:
        if r.source.branch == 'release':
            return r
    return requests[0]

CC'ing @siko1056 and @jweaton.