Update the github workflow to use the 18.04 image.

The 18.04 image worked when last used. So probably this'll fix
the CI problem.

.github/workflows/ci.yml

@@ -35,7 +35,7 @@ on:
 jobs:
   build-riscos:
     # The type of runner that the job will run on
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     outputs:
       version: ${{ steps.version.outputs.version }}