This updates the test env to test with Node 20 only, pr_check is also updated to use Node 20.
The changes shouldn't affect users in any way, please let me know if you think it might.
There were common async flakes when using testing-library with Node 18, those often surfaced in gh actions / pr_check only and did not reproduce locally unless resources got limited (I can reproduce some flakes in Power Saver mode only, leading me to conclusion, that it's just async being mischievous). Switching to testing with Node 20 only should also allow us to re-enable some tests that got disabled specifically because them running with Node 18 ended up in a time out.
It is now possible to run UI testing in ephemeral environment. This
script builds the frontend container, deploys image-builder with it's
frontend in ephemeral, runs the IQE test suite against that and reports
the result on the PR.
