make toolchains repo more reproducible (#4099)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

The versions.json file results from downloading a list of versions from
go.dev. (https://go.dev/dl/?mode=json)
This API is meant to be updated for every Go release and will result in
different outputs when rerun in the future.

By deleting the file, we make the behavior more reproducible: As long as
a specific version is requested, this should result in reproducible
behavior. (This assumes that go.dev does not replace released SDK
artifacts, which should be a fair assumption).

This should help other ecosystems (including Nix) with packaging Bazel,
since we want to depend on the output of `bazel vendor` (and pin it
against an expected hash for a given platform).

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

none

**Other notes for review**

go/private/sdk.bzl

@@ -91,6 +91,7 @@ def _go_download_sdk_impl(ctx):
         )
 
         data = ctx.read("versions.json")
+        ctx.delete("versions.json")
         sdks_by_version = _parse_versions_json(data)
 
         if not version: