gotip: explicitly set GOTOOLCHAIN=auto

Address issue 64665 by setting GOTOOLCHAIN=auto
if GOTOOLCHAIN env var is not explicitly configured
when invoking the real go binary from the gotip wrapper.
This ensures that user's global GOTOOLCHAIN setting
(set by `go env -w`) doesn't override gotip behavior.
Other version wrappers' behavior is left unchanged.

Fixes golang/go#64665

Change-Id: I076d34e88de8d149542085c494d4c1c31b7f379e
Reviewed-on: https://go-review.googlesource.com/c/dl/+/763260
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>