GitHub Actions: do not use macOS latest runner for now

GitHub is currently migrating its macos-latest runner to macOS 11:
actions/runner-images#4060

Unfortunately, installing VirtualBox and Vagrant on this new version of
macOS is not easy, and the current macos-11 runner does not support
running virtual machines using Vagrant. This issue is being fixed on
actions/runner-images#4010 .

Until this Pull Request is merged, use macos-10.15 runner instead of
macos-latest, to continue using Vagrant to run the SELinux testsuite in
a virtual machine.

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
Acked-by: Ondrej Mosnacek <omosnace@redhat.com>

.github/workflows/vm_testsuite.yml

@@ -6,7 +6,8 @@ jobs:
   vm_testsuite:
 
     # Use VirtualBox+vagrant on macOS, as described in https://github.com/actions/virtual-environments/issues/433
-    runs-on: macos-latest
+    # Use an old version of macOS until https://github.com/actions/virtual-environments/pull/4010 is merged.
+    runs-on: macos-10.15
 
     steps:
     - uses: actions/checkout@v2