project tests: fine-tune the check for whether we are in CI
project tests: fine-tune the check for whether we are in CI

Follow-up on commit 4274e0f42a9673df981dd7a7a456f4d4fcfe5452. We want to
allow tests to be skipped freely in third-party environments, so this
should check the jobname, not whether $CI exists.

We will anyways raise an error when trying to run the project tests, if
$CI is set but no jobname is set.

mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,pexip/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,pexip/meson