[Dune-devel] Nightly tests still using Ubuntu 18.04

[Christoph Grüninger]

Dear Andreas,
dear Simon,

our nightly tests still using Ubuntu 18.04. Currently dune-istl fails, 
probably because a change broke (with full intention) Clang++ 5.0.
You adjusted the CI images. Can you please have look there, too?

One of the failing build pipelines:
https://gitlab.dune-project.org/infrastructure/dune-nightly-test/-/pipelines/55585

Bye
Christoph

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