portal: Pass run-environ to the spawned flatpak process, not the sandbox

Instead of modifying the host-like run environment to clear the sandbox
environment, we'll use the new --clear-env flag which does the correct
thing.

Assisted-by: Claude:opus-4.6
Closes: #5271

portal/flatpak-portal.c

@@ -917,17 +917,6 @@ handle_spawn (PortalFlatpak         *object,
         }
     }
 
-  /* TODO: Ideally we should let `flatpak run` inherit the run environment
-   * of the instance, in case e.g. a LD_LIBRARY_PATH is needed to be able
-   * to run `flatpak run`, but tell it to start from a blank environment
-   * when running the Flatpak app; but this isn't currently possible, so
-   * for now we preserve existing behaviour. */
-  if (arg_flags & FLATPAK_SPAWN_FLAGS_CLEAR_ENV)
-    {
-      char *empty[] = { NULL };
-      env = g_strdupv (empty);
-    }
-  else
     {
       static const char * const mock_run_environ[] = { "FOO=bar", NULL };
 
@@ -965,6 +954,9 @@ handle_spawn (PortalFlatpak         *object,
 
   g_ptr_array_add (flatpak_argv, g_strdup ("run"));
 
+  if (arg_flags & FLATPAK_SPAWN_FLAGS_CLEAR_ENV)
+    g_ptr_array_add (flatpak_argv, g_strdup ("--clear-env"));
+
   sandboxed = (arg_flags & FLATPAK_SPAWN_FLAGS_SANDBOX) != 0;
 
   if (sandboxed)