Enable setting the base for a PR using labels

[connorjward]

This PR lets us set the base branch to main or release (and therefore use a dev/release build) using labels. This is useful for cases where you have dependent PRs and therefore the base branch may not be main or release.

[JHopeCollins]

Why do we need this now but we didn't before?

[connorjward]

I want the workflow to be triggered whenever the labels change, not just when a new label is added. I ran into this because I was switching out the labels to test this and found that it wasn't being rerun as expected.

[JHopeCollins]

Just to check that I understand. Is this for the case where I have this branch structure:

base -----------------> base
    \                  /
     branch1 -> branch2

and I have a PR to merge branch1 into branch2?

Currently we look at the target branch to determine whether to build firedrake using release or main process, but if the target is branch2 then we're stumped.
After this PR I will be able to use a label to say "I know my target is branch2 but this PR is branched from/will eventually end up in release (or main) so use the corresponding build process in CI".

Is that correct?

[connorjward]

Just to check that I understand. Is this for the case where I have this branch structure:

base -----------------> base
    \                  /
     branch1 -> branch2

and I have a PR to merge branch1 into branch2?

Currently we look at the target branch to determine whether to build firedrake using release or main process, but if the target is branch2 then we're stumped. After this PR I will be able to use a label to say "I know my target is branch2 but this PR is branched from/will eventually end up in release (or main) so use the corresponding build process in CI".

Is that correct?

Yep spot on.