[ci] fix ci concurrent runs
Diclaimer: this is not a DuMux issue but I have no idea, where to put problems with the ci-configuration.
The concurrency parameter of gitlab-runner larger than one can be a trouble maker as several processes work within one environment and can write each others resources. See https://docs.gitlab.com/ee/ci/runners/configure_runners.html#handling-concurrency . The configuration switches are also documented in https://docs.gitlab.com/runner/configuration/advanced-configuration.html . There is in our setting interaction with the slurm system status. A python script checks periodically how many slurm jobs are running and writes out a parameter adapted gitlab-runner config file.