cmd/go: fix the rebuilding stale packages test

Non-main packages do not depend on the "runtime" package,
but main packages still do. Use a main package in the test.

This change passes the -i flag to the install command
to allow installation of updated dependencies,
and removes "install std" as unnecessary.

https://golang.org/cl/107957 is relevant to fixed test.

Updates #24436

Change-Id: If1845f37581a16ad77e72e50be21010e198bc7c5
Reviewed-on: https://go-review.googlesource.com/103675


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>