CI: Rename tiny2040 to tiny2040_8mb.

It was not super obvious that this build is specific to the 8mb
version of Tiny 2040.

.github/workflows/micropython.yml

@@ -21,7 +21,7 @@ jobs:
           board: RPI_PICO
         - name: picow
           board: RPI_PICO_W
-        - name: tiny2040
+        - name: tiny2040_8mb
           board: PIMORONI_TINY2040
         - name: picolipo_4mb
           board: PIMORONI_PICOLIPO_4MB