cmd/go: always return the cached path from go tool -n
If we're running go tool -n always return the cached path of the tool.
We can't always use the cached path when running the tool because if we
copied the tool to the cached location in the same process and then try
to run it we'll run into #22315, producing spurious ETXTBSYs.
Fixes #72824
Change-Id: I81f23773b9028f955ccc97453627ae4f2573814b
Reviewed-on: https://go-review.googlesource.com/c/go/+/688895
Auto-Submit: Michael Matloob <matloob@golang.org> Reviewed-by: Michael Matloob <matloob@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>