Set MESON_CI_JOBNAME for all CI jobs which run project tests.
(Note that ${{ github.job }} is the literal job.id used in the yaml, not
any name given to the job with job.id.name, and so is the same for all
matrix entries, and thus not suitable for our purposes there).
* CI: Don't run cross-only test when other workflows are changed
* CI: Move Cygwin test run from Azure Pipelines to GitHub workflow
Also review installed package list, dropping detritus from 42792554, not
removed in f5ed85d7.
Add '--cross-only' option to run_tests.py, so we can arrange not to run
tests in the 'native' suite when only a cross-compiler is available, as
they can't succeed.