[testing] set -O1 in msan builds (#38118)

As title

Closes #38118

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38118 from apolcyn:set_flags 5f02df3058
PiperOrigin-RevId: 696195520

tools/remote_build/linux.bazelrc

@@ -62,6 +62,10 @@ build:msan --action_env=LD_LIBRARY_PATH=/usr/local/libcxx-msan/lib
 # the version having https://github.com/googleapis/google-cloud-cpp/pull/14151
 build:msan --cxxopt='-std=c++17'
 build:msan --host_cxxopt='-std=c++17'
+# Attempt to fix flakes on MSAN thought to be related to MSAN being too slow
+# in unoptimized builds.
+build:msan --copt=-O1
+build:msan --cxxopt=-O1
 
 # thread sanitizer: most settings are already in %workspace%/.bazelrc
 # we only need a few additional ones that are Foundry specific