cmd/go: convert TestExecutableGOROOT to the script framework

cmd/go: convert TestExecutableGOROOT to the script framework

Part of converting all tests to the script framework, but also
just working around flakiness when adding t.Parallel to
TestExecutableGOROOT.

Also, undo the changes to copyFile in golang.org/cl/220317 because
they didn't help.

Fixes #37306

Change-Id: I3348fa1d8f2589febe89604ac19a21df20075c4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/220319
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>