travis: Enable caching for sonarcloud scan

.travis.yml

@@ -41,6 +41,9 @@ jobs:
       if: type = push AND env(SONAR_TOKEN) IS present
       git:
         depth: false
+      cache:
+        directories:
+          - $HOME/.sonar-cache
       addons:
         sonarcloud:
           organization: "strongswan"

scripts/test.sh

@@ -433,6 +433,8 @@ sonarcloud)
 		-Dsonar.projectVersion=$(git describe)+${TRAVIS_BUILD_NUMBER} \
 		-Dsonar.sources=. \
 		-Dsonar.cfamily.threads=2 \
+		-Dsonar.cfamily.cache.enabled=true \
+		-Dsonar.cfamily.cache.path=$HOME/.sonar-cache \
 		-Dsonar.cfamily.build-wrapper-output=bw-output || exit $?
 	rm -r bw-output .scannerwork
 	;;