Don't rely on __noChroot for corepkgs

This doesn't work anymore if the "strict" chroot mode is
enabled. Instead, add Nix's store path as a dependency. This ensures
that its closure is present in the chroot.

corepkgs/unpack-channel.nix

@@ -37,6 +37,5 @@ derivation {
   # No point in doing this remotely.
   preferLocalBuild = true;
 
-  # Don't build in a chroot because Nix's dependencies may not be there.
-  __noChroot = true;
+  inherit chrootDeps;
 }