tasks: Fix and simplify bots checkout
tasks: Fix and simplify bots checkout

Commit 8b27da1eb44ff had a bug where the *first* cockpit-tasks round
always checked out the default branch of bots, instead of
`$COCKPIT_BOTS_BRANCH`. This led to bots PR failures when running
against an outdated master branch from a clone (as they usually are).
Fix that by using the `clone -b` option.

The bots checkout is now only for cockpit-tasks's benefit -- the
individual tasks and tests all check out their own bots. Thus we can
simplify `update_bots()` and only differ between "new checkout for the
first time" and "pull to get the latest version".

cockpit-project/cockpituous,cockpit-project/cockpituous