Remove the Bazel provided default cxxopt -std=c++0x and replace it

with -std=c++14 when testing with GCC 5.

PiperOrigin-RevId: 458502826
Change-Id: I83637a222b5c0a54e55d93ce7fdbc878af27b250

ci/linux-presubmit.sh

@@ -72,6 +72,7 @@ time docker run \
   --workdir="/src" \
   --rm \
   --env="CC=/usr/local/bin/gcc" \
+  --env="BAZEL_CXXOPTS=-std=c++14" \
   ${LINUX_GCC_FLOOR_CONTAINER} \
     /usr/local/bin/bazel test ... \
       --copt="-Wall" \