nix: set primary and secondary groups for nixbldN.

Sounds superfluous, but it works.

srcpkgs/nix/template

@@ -1,7 +1,7 @@
 # Template file for 'nix'
 pkgname=nix
 version=1.11.4
-revision=4
+revision=5
 build_style=gnu-configure
 # Use /nix/var as suggested by the official Manual.
 configure_args="--localstatedir=/nix/var"
@@ -17,6 +17,7 @@ system_groups="nixbld"
 for f in {1..9}; do
 	system_accounts+=" nixbld${f}"
 	eval export nixbld\${f}_pgroup="nixbld"
+	eval export nixbld\${f}_groups="nixbld"
 done
 # Create required run-time directories.
 make_dirs="