Use apt-get to install Clang on CI

.github/workflows/osrm-backend.yml

@@ -535,7 +535,7 @@ jobs:
         # we only enable lto for release builds
         # and therefore don't need to us ld.gold or llvm tools for linking
         # for debug builds
-        if [[ ${BUILD_TYPE} == 'Release' ]]; then
+        if [[ "${CCOMPILER}" = clang-* && ${BUILD_TYPE} == 'Release' ]]; then
           ${MASON} install binutils 2.27
           echo "$(${MASON} prefix binutils 2.27)/bin" >> $GITHUB_PATH
         fi