cmd/go: pass GOEXPERIMENT through to subtests

cmd/go: pass GOEXPERIMENT through to subtests

This fixes:

	export GOEXPERIMENT=unified
	go install cmd
	go install std cmd
	go install std cmd
	go test -short cmd/go -run=TestScript/test_relative_import_dash_i

That script test checks that runtime is non-stale, but whether it's stale
depends on the setting of GOEXPERIMENT. Stop filtering that variable out.

Change-Id: I71bdbca495c16981cdcddf4ab4d87a38ca72a389
Reviewed-on: https://go-review.googlesource.com/c/go/+/412874
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>