firefox: restore compatibility for extraPolicies

This commit makes it possible to specify Firefox' extraPolicies
through:

    programs.firefox.package = pkgs.firefox.override {
      extraPolicies = {... }
    }

This was possible in the past but was broken by:

  3feeb77
  firefox: add support for specifying policies (#4626)

modules/programs/firefox.nix

@@ -172,7 +172,7 @@ let
     else if versionAtLeast config.home.stateVersion "19.09" then
       package.override (old: {
         cfg = old.cfg or { } // fcfg;
-        extraPolicies = cfg.policies;
+        extraPolicies = (old.extraPolicies or { }) // cfg.policies;
       })
     else
       (pkgs.wrapFirefox.override { config = bcfg; }) package { };

tests/modules/programs/firefox/policies.nix

@@ -9,18 +9,28 @@
     programs.firefox = {
       enable = true;
       policies = { BlockAboutConfig = true; };
+      package = pkgs.firefox.override {
+        extraPolicies = { DownloadDirectory = "/foo"; };
+      };
     };
 
     nmt.script = ''
       jq=${lib.getExe pkgs.jq}
       config_file="${config.programs.firefox.finalPackage}/lib/firefox/distribution/policies.json"
 
       assertFileExists "$config_file"
+
       blockAboutConfig_actual_value="$($jq ".policies.BlockAboutConfig" $config_file)"
 
       if [[ $blockAboutConfig_actual_value != "true" ]]; then
         fail "Expected '$config_file' to set 'policies.BlockAboutConfig' to true"
       fi
+
+      downloadDirectory_actual_value="$($jq ".policies.DownloadDirectory" $config_file)"
+
+      if [[ $downloadDirectory_actual_value != "\"/foo\"" ]]; then
+        fail "Expected '$config_file' to set 'policies.DownloadDirectory' to \"/foo\""
+      fi
     '';
   };
 }