cmd/go/internal/cache: use internal/godebug for three GODEBUGs

cmd/go/internal/cache: use internal/godebug for three GODEBUGs

And register/document them.

Change-Id: If0f1cf3c09230e0f63d03c52e56e51a030468ab2
Reviewed-on: https://go-review.googlesource.com/c/go/+/487655
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>