fix rootless Buildah mount prob. in Github Actions

Due to a recent change in Buildah and a configuration error in the Github
virtual environment, Buildah cannot mount layers in rootless mode. This patch
fixes the problem by explicitly specifying the correct mount program to use, as
suggested in [this bug
report](actions/runner-images#3080).

.github/workflows/test.yaml

@@ -134,6 +134,9 @@ jobs:
     name: Build container image
     needs: build-python-package
     runs-on: ubuntu-20.04
+    env:
+      # Fixes https://github.com/actions/virtual-environments/issues/3080
+      STORAGE_OPTS: overlay.mount_program=/usr/bin/fuse-overlayfs
     steps:
       - name: Checkout
         uses: actions/checkout@v2