Enable runner-specific make concurrency settings

Using fixed make concurrency settings on all runners is not flexible and
requires .gitlab-ci.yml to be modified each time tweaking these settings
is needed.  Use environment variables which are expected to be set by
the runner (defaulting to 1 in case they are not set) for controlling
make concurrency.
7 jobs for 138-tweak-ci-settings in 6 minutes and 34 seconds (queued for 2 seconds)