Allow useful CI to run in forks

Allow useful CI to run in forks

* Move the fast jobs with no dependencies to the first stage.
* Remove the global KTH-specific job runner tag from jobs in the pre-build stage.
* Use the `pre-build` stage as the dependency for all later stages, rather than the `simple-build` job, specifically.
* Convert rule sets to new *rules* syntax.
* Use '$CI_PROJECT_NAMESPACE == "gromacs"' to distinguish jobs created with access to GROMACS GitLab infrastructure.

Fixes #3458