[Feat] Add a clang build to Github CI actions

[nilason]

Is your feature request related to a problem? Please describe.
Using clang to build GRASS GIS is in my experience in general no problem. However, it issues a range of warnings (by default settings, see #1247) not always seen by e.g. gcc builds, which would be good to be able to catch.
There is also a possibility, however unlikely, the compilation may fail.

Describe the solution you'd like
Now that ubuntu-16.04 is dropped (#1254), there may be a slot available for a clang-on-linux build (if technically feasible).

[nilason]

Implemented with #2661.