Release 2.0.13 and update bazel toolchain (#694)

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

scripts/test_bazel.sh

@@ -13,7 +13,7 @@ VER=$(grep -oP '(?<=latest\": \")([\d\.]+)(?=\")' \
 HASH=$(grep "${VER}" emscripten-releases-tags.txt \
       | grep -v latest \
       | cut -f4 -d\")
-      
+
 FAILMSG="!!! scripts/update_bazel_toolchain.sh needs to be run !!!"
 
 # Ensure the WORKSPACE file is up to date with the latest version.