Don't use ccache in Ubuntu 18.04 CI job

It doesn't work there because the current Node used by this action
doesn't support Ubuntu 18.04.

See actions/checkout#1442 (for the same issue
in another action).

Author: vadz

.github/workflows/ci.yml

@@ -96,6 +96,7 @@ jobs:
 
       - name: Install CCache
         uses: hendrikmuhs/[email protected]
+        if: matrix.container != 'ubuntu:18.04'
         with:
           key: ${{ matrix.name }}