tests: don't override scraping of nixpkgs for thunderbird suite

Instead, pick just the required packages to pass the tests.

Note: we have to refer to real derivations because symlinkJoin assumes
the passed derivation is a directory with files, otherwise it fails as:

@thunderbird@: No such file or directory

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

tests/modules/programs/thunderbird/thunderbird-with-firefox.nix

@@ -1,5 +1,9 @@
 # Confirm that both Firefox and Thunderbird can be configured at the same time.
 { lib, realPkgs, ... }:
 lib.recursiveUpdate (import ./thunderbird.nix { inherit lib realPkgs; }) {
-  programs.firefox.enable = true;
+  programs.firefox = {
+    enable = true;
+    # Darwin doesn't support wrapped Firefox, using unwrapped instead
+    package = realPkgs.firefox-unwrapped;
+  };
 }

tests/modules/programs/thunderbird/thunderbird.nix

@@ -1,8 +1,6 @@
 { lib, realPkgs, ... }: {
   imports = [ ../../accounts/email-test-accounts.nix ];
 
-  _module.args.pkgs = lib.mkForce realPkgs;
-
   accounts.email.accounts = {
     "hm@example.com" = {
       thunderbird = {