patch(_update_bundle.yaml): Alphabetically sort snap revision list

[lucasgameiroborges]

Issue

The workflow mistakenly created PRs for updating the snaps list because of ordering, even though the revisions itself didn't change. This is because, for python:

> {"packages": ["snap1", "snap2", "snap3"]} != {"packages": ["snap1", "snap3", "snap2"]}
True

then, snaps != old_snaps was being set to true every time, and the output of updates_available was mistakenly being set to true

Solution

Sort the snaps list. See test runs:

postgresql: canonical/postgresql-bundle@d5056b5
mysql: canonical/mysql-bundle@4269833