Lock build to macOS 10.15

The macos-latest label  moved to macOS 11. However, this image from
GitHub doesn't have VirtualBox installed, and it's not clear that this
will ever happen. 10.15 is still supported, though.

These two issues are provided as reference:

- actions/runner-images#4060
- actions/runner-images#4010

.github/workflows/build.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 jobs:
   packer:
     name: "Build appliance using Packer"
-    runs-on: macos-latest
+    runs-on: macOS-10.15
     permissions:
       contents: write