Change one CI job to use an editable install

[rgommers]

This is a follow-up to gh-26325, which fixed the test suite so it passes (at least on a regular Linux numpy-dev env) with an editable install:

$ pip install -e . --no-build-isolation
$ pytest numpy

This is not tested at all in CI at the moment, so it's fairly easily broken again. We should pick a suitable existing job and change it to use an editable install rather than spin or a regular install via a wheel. In case there's no suitable job, we can add a new one - but that's probably not necessary. Just need a generic Linux one for which compile tests (which don't work with editable installs) aren't important.