Set Percy's target branch to the PR base [v4.x]

[querkmachine]

Modify GitHub actions config so that Percy runs comparisons against the PR's base branch rather than always using main.

This is particularly needed on the v4 support branch as main has diverged quite significantly, however we still intend to make v4 releases and want visual regression tests to work as expected.

[colinrotherham]

Looks like it's picking up diffs from some commits we backported

df2c476 for main
d3c249f for support/4.x

Which is good?

[querkmachine]

The Percy changes seem good. The disappearance of margin above the header/footer examples would be consistent with the removal of the main wrapper from the examples. 👍

There's a tiny change on the select component where it appears to have become a couple of pixels narrower between different versions of Edge, but as there's no change in Firefox I would assume this change is not a result of changes to our code.

[colinrotherham]

Works great in a manual run too? Let's do it

[querkmachine]

@colinrotherham From what I can gather there isn't an obvious way of making this work in manual runs without hardcoding the branch name. GitHub Actions always defaults to using main as github.event.pull_request.base is presumably null or undefined. Alternatively, manual runs always use the version of the file in the main branch, so aren't using this branch's configuration anyway.

Choosing from the "Use workflow from" dropdown in the Actions interface only changes what branch is compared against main.

I've merged on the basis that the last commit to this branch seemed to produce the expected comparisons in Percy: https://percy.io/55ba5e1a/govuk-frontend/builds/29991587/