gnupgMinimal: init

This used to be a non-exposed part of the systemd derivation. However,
in #509324 this was removed. Re-add this to use it for systemd-import
and systemd-sysupdate to provide a minimal gnupg without bash. This
fixes the bashless tests.

pkgs/top-level/all-packages.nix

@@ -2358,6 +2358,10 @@ with pkgs;
     pinentry = if stdenv.hostPlatform.isDarwin then pinentry_mac else pinentry-gtk2;
   };
   gnupg = gnupg24;
+  gnupgMinimal = gnupg.override {
+    enableMinimal = true;
+    guiSupport = false;
+  };
 
   gnused = callPackage ../tools/text/gnused { };