chore: revert from macos-11 (latest) to macos-10.15

The `macos-latest` image was recently updated to macos 11 which does
not include virtualbox anymore.
This commit pins the previous version macos-10.15 until a more permament
fix can be found.
More information can be found in the discussion of this PR:
#840 (comment)
and in the following issue and PR for the actions/virtual-environments:
actions/runner-images#4060
actions/runner-images#4010

Co-authored-by: Markus Wolf <mail@markus-wolf.de>

.github/workflows/checks.yml

@@ -93,7 +93,7 @@ jobs:
   test-macos:
     needs: dump_images
     name: test-macos
-    runs-on: macos-latest
+    runs-on: macos-10.15
     continue-on-error: true # Don't let macos test fail whole workflow
     env:
       ISO_PATH: ~/.docker/machine/cache/boot2docker.iso