Fix compiler flags in Linux presubmit

CXX_FLAGS should be CXXFLAGS and the quoting was wrong.
As a result, "-Werror -Wdeprecated" was not being applied.

https://cmake.org/cmake/help/latest/envvar/CXXFLAGS.html

PiperOrigin-RevId: 506656655
Change-Id: Ic5e861be3b9c32257eb9aabb845c931f3cba7122

ci/linux-presubmit.sh

@@ -51,7 +51,7 @@ for cc in /usr/local/bin/gcc /opt/llvm/clang/bin/clang; do
       --workdir="/build" \
       --rm \
       --env="CC=${cc}" \
-      --env="CXX_FLAGS=\"-Werror -Wdeprecated\"" \
+      --env=CXXFLAGS="-Werror -Wdeprecated" \
       ${LINUX_LATEST_CONTAINER} \
       /bin/bash -c "
         cmake /src \