Ensure tests are run for PRs targeting staging to prevent regressions

[Greenheart]

Several times, we've been caught by failing tests only when merging to main (which is before deploying to production).

It would save a lot of time and frustration if tests were run against PRs targeting staging too, to catch these errors as early as possible and making production deployments easier.

To fix this, we probably just need to add staging to the list of target branches in

klimatkollen/.github/workflows/pr_check.yml

Lines 5 to 7 in 392fb47

	branches:
	- main