github/workflows: set build mode to manual

This seems to be recommended by the documentation.

.github/workflows/lone.yml

@@ -77,10 +77,12 @@ jobs:
       actions: read
       contents: read
       security-events: write
+
     strategy:
       fail-fast: false
       matrix:
-        language: [ 'cpp' ]
+        language: c-cpp
+        build-mode: manual
 
     steps:
     - name: Checkout lone repository