CI: Build on Ubuntu 18.04, use newer clang-format

There are too many issues with 20.04 to successfully build with
VirtualCam - the azure kernel is missing videodev headers. For now,
use 18.04 LTS directly for main CI builds.

Both 18.04 and 20.04 include clang-format-10 without issue.

.github/workflows/main.yml

@@ -352,7 +352,7 @@ jobs:
           path: ./release/*.dmg
   ubuntu64:
     name: 'Linux/Ubuntu 64-bit'
-    runs-on: [ubuntu-latest]
+    runs-on: [ubuntu-18.04]
     steps:
       - name: 'Checkout'
         uses: actions/checkout@v2.3.3