System tests: More robust setup of DUNE modules

[MakisH]

• Split the git clones into multiple Docker run steps
• Retry if a git clone fails, for three times, with delay

I am only adding this to the Ubuntu 24.04 Dockerfile, since we anyway don't have this in 22.04.

Workaround (partial fix) for #768

GitHub Actions run: https://github.com/precice/tutorials/actions/runs/25845382737

Note that the two-scale-heat-conduction test is known to fail at the fieldcompare test. I am executing the dumux-test instead of the release-test, for convenience.

[Copilot]

Pull request overview

This PR makes the Ubuntu 24.04 system-test Docker setup for DuMuX/DUNE more resilient to transient clone failures, aligning with the workaround requested for network-related artifact retrieval failures.

Changes:

• Splits the DUNE/DuMuX repository clones into separate Docker RUN layers.
• Adds intended retry behavior with delay and cleanup around each git clone.
• Keeps the subsequent DuMuX adapter build flow unchanged.

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[Copilot]

Pull request overview

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[Copilot]

Pull request overview

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