Tell Drone to use CLang.

.drone.yml

@@ -5,12 +5,18 @@ type: docker
 steps:
   - name: test-debug-linux
     image: deukhoofd/linux64builder
+    environment:
+      CC: /usr/bin/clang
+      CXX: /usr/bin/clang++
     commands:
       - cmake -DCMAKE_BUILD_TYPE=Debug . -B build-debug
       - cmake --build build-debug --target all -- -j 4
       - build-debug/CreatureLibTests -s --durations yes --use-colour yes
   - name: test-release-linux
     image: deukhoofd/linux64builder
+    environment:
+      CC: /usr/bin/clang
+      CXX: /usr/bin/clang++
     commands:
       - cmake -DCMAKE_BUILD_TYPE=Release . -B build-release
       - cmake --build build-release --target all -- -j 4