CI: Add clang to GitHub Actions build-tests matrix (#1812)

CI: Add clang to GitHub Actions build-tests matrix (#1812)

Add "compiler" dimension to build-tests matrix.

Do not test layer-04-noauth-everything to partially compensate for the
new matrix dimension repeating other layer tests 3 times.

Change build-tests artifacts name pattern to include compiler dimension
to avoid name clashes that would result in logs being overwritten and
lost. Also use target environment instead of runner environment in
anticipation of adding container-based tests where the two differ.