CI: Include WP trunk in e2e test runs on PRs

[ockham]

I've noticed that e2e tests are failing when run against WP trunk. (See e.g. here and here.)

This is somewhat unsurprising, since we're currently near the release of WP 7.0, so WP trunk has evolved considerably since the latest stable release (6.9.x).

It'd be good to preemptively catch problems, so I'm adding back WP trunk to the test matrix for PRs (rather than only on merge, i.e. after the fact).

If we find that e2e tests take to long, we might want to audit them (and e.g. tweak test fixtures and setup, and/or use test.step to cover multiple scenarios in one test).

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.99%. Comparing base (e6bb412) to head (cba18aa).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #373      +/-   ##
============================================
- Coverage     53.88%   52.99%   -0.89%     
  Complexity     4419     4419              
============================================
  Files           298      298              
  Lines         39448    39448              
============================================
- Hits          21256    20906     -350     
- Misses        18192    18542     +350     

Flag	Coverage Δ
e2e-js	45.71% <ø> (-0.02%)	⬇️
e2e-php	41.05% <ø> (-1.30%)	⬇️
javascript	15.07% <ø> (ø)
phpunit	29.96% <ø> (ø)

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