gRPC 1.29.0 can not be built with our precompiled libs because it
accidentally overrides the C++ standard specified by the users.
This was fixed in 1.30.0.
Change-Id: I9a0390b3f5ec8dae6c295562f3a94b3dfa681dd1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/925
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>