Switch from LLVM source-based instrumentation back to gcov
Switch from LLVM source-based instrumentation back to gcov

I.e. from -Cinstrument-coverage to -Zprofile

This makes grcov take a long amount of time to run, hmmm...

Part-of: <https://gitlab.gnome.org/GNOME/librsvg/-/merge_requests/724>

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg