ci: Use new mbed-os-env docker image in "multiple executables" test

The github workflow to test building multiple executables was using the
soon to be deprecated docker image "mbedos/mbed-os-env:latest". This
commit points it at the new docker image on ghcr.io.

Author: rwalton-arm

.github/workflows/test_building_multiple_executables.yml

@@ -5,7 +5,7 @@ on: [pull_request]
 jobs:
   multiple-executables-example:
     runs-on: ubuntu-latest
-    container: mbedos/mbed-os-env:latest
+    container: ghcr.io/armmbed/mbed-os-env:master-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v2