Remove sgid from .nix-portable and set group

This should prevent issues when putting it under
/gpfs/{projects,scratch} that have sgid and group=nobody.

pkgs/nix-portable/default.nix

@@ -150,6 +150,15 @@ let
     [ -z "\$NP_LOCATION" ] && NP_LOCATION="\$HOME"
     NP_LOCATION="\$(readlink -f "\$NP_LOCATION")"
     dir="\$NP_LOCATION/.nix-portable"
+
+    # Create NP_LOCATION and remove sgid bit
+    mkdir -p \$dir
+    if [ ! -z "\$BSC_MACHINE" ]; then
+      # Attempt to avoid issues with sgid folders
+      chmod g-s \$dir
+      chgrp bsc \$dir
+    fi
+
     store="\$dir/nix/store"
     # create /nix/var/nix to prevent nix from falling back to chroot store.
     mkdir -p \$dir/{bin,nix/var/nix,nix/store}