Invalidate go_repository_cache when host env vars change (#2183)

**What type of PR is this?**

Bug fix

**What package or component does this PR mostly affect?**
go_repository


**What does this PR do? Why is it needed?**
This PR calls `repository_ctx.getenv` to get
`GO_REPOSITORY_USE_HOST_CACHE` and all Go env vars, so when these
variables changes, the go_repository_cache is invalidated and updated.

**Which issues(s) does this PR fix?**

Fixes #2182

**Other notes for review**