Use the prior layer's openssl

This may make no difference but we're not doing anything to override
openssl_1_1 in this overlay so we don't need to get it from `self`.

overlays.nix

 self: super: {
-  openssl = self.openssl_1_1;
+  openssl = super.openssl_1_1;
 
   ristretto = super.callPackage ./ristretto.nix { };