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).
Fixes a test when using pkgconf instead of pkg-config.
The .pc file in the pkgconfig-gen test requires it and pkgconf seems be
stricter here and fails to validate.