python: Set NIX_PYTHONPATH in check phase

package namespaces are implemented in pip with *.pth files. These files
are only processed in "site" directories. The PYTHONPATH directories
aren't automatically considered "site" directories. NIX_PYTHONPATH
directories are added to the "site" directories by
pkgs/development/interpreters/python/sitecustomize.py.

pkgs/development/interpreters/python/hooks/setuptools-check-hook.sh

@@ -6,7 +6,7 @@ setuptoolsCheckPhase() {
     runHook preCheck
 
     cp -f @setuppy@ nix_run_setup
-    @pythonCheckInterpreter@ nix_run_setup test
+    NIX_PYTHONPATH="$PYTHONPATH" @pythonCheckInterpreter@ nix_run_setup test
 
     runHook postCheck
     echo "Finished executing setuptoolsCheckPhase"