Gitlab CI: make shared runners work
As seen in !421 (closed), selecting only acts runners for the compilation CI jobs results in contributors working on their own forks being unable to complete a CI pipeline.
This MR updates the env script and the CI config to set the -jN
to either 8 when it detects it's running in one of our runners, or 2 if it's running on a shared runner.
This should resolve this problem.