Upstream cmake build downloads opencensus-proto

[h-vetinari]

Seen in #213 on all platforms (see third line)

-- Found OpenSSL: /Users/runner/miniforge3/conda-bld/grpc-split_1660634755701/_build_env/lib/libcrypto.dylib (found version "3.0.5")  
-- Found ZLIB: /Users/runner/miniforge3/conda-bld/grpc-split_1660634755701/_build_env/lib/libz.dylib (found version "1.2.12") 
-- Downloading from https://storage.googleapis.com/grpc-bazel-mirror/github.com/census-instrumentation/opencensus-proto/archive/v0.3.0.tar.gz, if failed, please try configuring again
-- Configuring done
-- Generating done

We should replace this with builds from https://github.com/conda-forge/opencensus-proto-feedstock.

[h-vetinari]

It seems there's a circular dependency, because opencensus depends on grpcio, which depends on libgrpc...