Allow nightly CI to fail

Author: adamgreig

cortex-m-rt/.github/workflows/ci.yml

@@ -8,6 +8,7 @@ name: Continuous integration
 jobs:
   ci-linux:
     runs-on: ubuntu-20.04
+    continue-on-error: ${{ matrix.experimental || false }}
     strategy:
       matrix:
         # All generated code should be running on stable now