shell.nix: put only direct deps in the PATH
shell.nix: put only direct deps in the PATH

This shortens the PATH and also fixes the problem that multiple
Python interpreters are present in the PATH. The only interpreter
is now the one with the correct PYTHONPATH so we don't need to add
it to the PYTHONPATH manually.

dpausp/arguments,dpausp/arguments,dpausp/arguments,dpausp/arguments