msan_builder: Inject MSan flags in wrapper if they don't exist.
msan_builder: Inject MSan flags in wrapper if they don't exist.

Some packages build test binaries that don't use DEB_C(XX)FLAGS and fail
during linking.

robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz,robertswiecki/oss-fuzz