If we aren't using GOMA/RBE, don't specify the number of jobs.

Explicitly using the number of cores is not necessarily the right choice
for this argument. In this case, just let ninja choose the right value.

Change-Id: Ia236dc98266eba49efca28ca83eecf1382cbec9a
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/83400
Reviewed-by: John McDole <codefu@google.com>
Commit-Queue: Jackson Gardner <jacksongardner@google.com>