use ccache for jenkins/linux_clang_dbgutil_64's compilerplugins

Change-Id: I4a4c4ee111d6a22c899ae0d288ae04d760ec996a
Reviewed-on: https://gerrit.libreoffice.org/65272
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>

distro-configs/Jenkins/linux_clang_dbgutil_64

 --enable-dbgutil
 --enable-compiler-plugins
-COMPILER_PLUGINS_CXX=g++ -std=c++11
+COMPILER_PLUGINS_CXX=ccache g++ -std=c++11