[rush] Add a `RUSH_BUILD_CACHE_OVERRIDE_JSON` env var to override the build cache configuration.

[iclanton]

Summary

This PR adds support for a RUSH_BUILD_CACHE_OVERRIDE_JSON environment variable that allows users to override the build cache configuration.

How it was tested

Tested by setting up and pulling cache from a Gradle server in this repo.

Impacted documentation

https://rushjs.io/pages/configs/environment_vars/#docusaurus_skipToContent_fallback