Upload coverage HTML reports (#1421)

Uploads two reports to Buildkite, one from cargo-cov and one from lcov via grcov.  The lcov one is busted on linux and is what we need to bring codecov.io back up again. It works great on macos if you wanted to generate them locally and prefer lcov HTML reports.

* Also comment out non-coverage build to speed things up.

ci/docker-rust/Dockerfile

@@ -13,6 +13,7 @@ RUN set -x && \
     apt install -y \
       buildkite-agent \
       cmake \
+      lcov \
       libclang-common-7-dev \
       llvm-7 \
       rsync \