CI: Continue other MicroPython builds if one fails.

In almost all cases it's more useful to know if a given build
is likely to succeed rather than have *everything* fail. This
change adjusts the workflow to allow other builds to continue
if one fails.

.github/workflows/micropython.yml

@@ -13,6 +13,7 @@ jobs:
   build:
     name: ${{ matrix.name }} (${{ matrix.board }})
     runs-on: ubuntu-20.04
+    continue-on-error: true
     strategy:
       matrix:
         include: