Adds cmd/cigocacher as the client to cigocached for Go caching over
HTTP. The HTTP cache is best-effort only, and builds will fall back to
disk-only cache if it's not available, much like regular builds.
Not yet used in CI; that will follow in another PR once we have runners
available in this repo with the right network setup for reaching
cigocached.
Updates tailscale/corp#10808
Change-Id: I13ae1a12450eb2a05bd9843f358474243989e967
Signed-off-by: Tom Proctor <tomhjp@users.noreply.github.com>
6637003cc8