github/workflows: Run unix port workflow if mpy-cross changes.

Otherwise there is no CI for mpy-cross if only it changes.

Signed-off-by: Damien George <damien@micropython.org>

.github/workflows/ports_unix.yml

@@ -10,6 +10,7 @@ on:
       - 'extmod/**'
       - 'lib/**'
       - 'examples/**'
+      - 'mpy-cross/**'
       - 'ports/unix/**'
       - 'tests/**'