Update from deprecated arduino:mbed platform in "smoke test" sketch compilation CI workflow

As of the 2.0.0 release of the Arduino Mbed OS Boards platform, the platform has been split into three separate
platforms, one for each class of boards. The platform for the Portenta boards is now arduino:mbed_portenta.

Author: per1234

.github/workflows/compile-examples.yml

@@ -35,7 +35,7 @@ jobs:
 
       matrix:
         board:
-          - fqbn: arduino:mbed:envie_m7
+          - fqbn: arduino:mbed_portenta:envie_m7
 
     steps:
       - name: Checkout