Use apt-get to install Clang on CI

.github/workflows/osrm-backend.yml

@@ -529,7 +529,9 @@ jobs:
         echo "$(${MASON} prefix ccache ${CCACHE_VERSION})/bin" >> $GITHUB_PATH
 
         # clang
+        if [[ "${CCOMPILER}" = "clang-6.0"]]; then
           sudo apt-get update -y && sudo apt-get install clang++-6
+        fo
         # we only enable lto for release builds
         # and therefore don't need to us ld.gold or llvm tools for linking
         # for debug builds