workflows/build: build all the basics in a single workflow

This new workflow builds both manuals, the shell and the lib tests all
in a matrix of four jobs. This allows re-using the shared checkout and
the pinned nixpkgs download and saves time in the most likely cache: No
changes, just download from cache. Each step checks the cancelled
condition, which causes it to run even if the previous steps failed.
This way we get a full picture even if the first step fails immediately.

This could later be optimized to build more in parallel as well, but
we'll first need to clear the conditions on building the manuals on the
master branch only.

This reduces the number of jobs from up to 8 to 4 for this part.

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