This change comes with fairly major change to the bazel toolchain.
- Use pre-built cache that comes with emsdk
- Mark cache as readonly using FROZEN_CACHE
- Pass `--sysroot` to match upstream emscripten change
* update bazel workspace for emscripten 2.0.3
* test to make sure WORKSPACE is up to date
* make it clear what happened when WORKSPACE is out of date
* intentionally add error to make sure circleci fails properly
* revert intentionally bad commit
* increase visibility of failure message
Co-authored-by: Mitch Foley <mitchfoley@google.com>
