Investigate usage of github actions/triggers to only run some CI steps on certain branches

[DominicRoyStang]

The current solution for filtering on branches is not optimal since we have to write multiline bash scripts to check the current branch. We also still have to pull images for steps we end up skipping (particularly long for the deploy step since we use the Helm community builder.

As per the comment in cloudbuild.yml, this issue exists to be able to filter steps in cloudbuild.yml based on the target branch. However, this Google Bug Tracker Issue suggests to follow this guide and use a trigger.yml file instead.

I'll need to look into how this is different from GitHub actions & whether we can ensure the following:

• Deploy steps are only run on master
• Deploy steps don't run unless the previous ones (eg. tests/linting) passed