Systemtests: checkout named branches

[MakisH]

We are currently restricted in checking out specific tags or commits, due to our caching implementation. It would be very convenient if we could also reliably checkout branches.

I mean there are easy ways to do this:

• Manipulate the Build_argument and append the current day and be okay if develop might be a day old?
• Have a cronjob invalidate the build cache in a certain interval.
  etc.

I think using commit-ish objects is the way to go here in the long run.
Only if we see, that we rebuild a lof of the layers often or spend more time building the container than running the simuations, we might need to think about it. We then can think of trading off some consistency for performance.

https://stackoverflow.com/questions/23303549/what-are-commit-ish-and-tree-ish-in-git

Originally posted by @valentin-seitz in #410 (comment)

[MakisH]

I think the easiest way to work around this is to have an option in the system tests that automatically gets the latest commit of each branch. This is what we now do in the OpenFOAM adapter CI, using the Get latest commit Action.