fix: python: remove spurious --ignore-installed

This arg was inadvertently added to pip install in nix-community#397, which breaks
transitive dependencies on the packages we custom-install, like
setuptools and wheel.

Per @phaer, it wasn't supposed to be there and has already been removed
in the next set of larger changes, but this should fix those transitive
dependencies in the meantime.

src/subsystems/python/builders/simple-python/default.nix

@@ -68,7 +68,6 @@
           --no-warn-script-location \
           --prefix="$out" \
           --no-cache \
-          --ignore-installed \
           . \
           $pipInstallFlags
       '';