ci: Update pipeline rules

The `autobot` runs in a scheduled pipeline from the main branch,
creating Merge Requests to the `main` branch. The `main` branch,
at any point in time, has already passed all CI checks, so we do
not need to run the same CI checks on the `main` branch again.

Signed-off-by: Saheer Babu <saheer.babu@arm.com>