Update branch filter regex

We intend to publish snaps for pre-release branches, which have
the following format: auotupdate/v1.33.0-alpha.0.

In order to do so, we need to update the "TIP_BRANCH" regex, which
is used to filter branches.

We'll need to clarify what "TIP_BRANCH" means and add an inline
comment.

scripts/util/util.py

@@ -9,7 +9,7 @@
 LOG = logging.getLogger(__name__)
 SNAP_NAME: str = "k8s"
 SNAP_REPO: str = "https://github.com/canonical/k8s-snap.git/"
-TIP_BRANCH = re.compile(r"^(?:main)|^(?:release-\d+\.\d+)$")
+TIP_BRANCH = re.compile(r"^(?:main)|^(?:release-\d+\.\d+)$|^(?:autoupdate\/v\d+\.\d+\.\d+-(?:alpha|beta|rc).\d+)$")
 
 
 def flavors(dir: str) -> list[str]: