thunderbird: use foldl' for profiles.ini merge

profilesIni folded recursiveUpdate over the profile list with the
non-strict foldl, building a thunk chain over the growing accumulator.
Use the strict foldl'.

modules/programs/thunderbird.nix

@@ -60,7 +60,7 @@ let
   profilesWithId = lib.imap0 (i: v: v // { id = toString i; }) (attrValues cfg.profiles);
 
   profilesIni =
-    lib.foldl lib.recursiveUpdate
+    lib.foldl' lib.recursiveUpdate
       {
         General = {
           StartWithLastProfile = 1;