separate builds per job to handle concurrency

Some hints on how to solve the problem in a way that is recommended by GitLab are here https://docs.gitlab.com/ee/ci/runners/configure_runners.html#handling-concurrency.

The examples suggest to use variables in the job definition, but hopefully it will be possible to do it at the executor level (there might be issues with automatic CUSTOM_ENV_* variable renaming).

The simple attempt of defining builds_dir as $CI_BUILDS_DIR/$CI_JOB_ID in the configure_exec stage does not seem to work. This is very likely related to #23