test: avoid touching GOOS/GOARCH in codegen driver

This change modifies the codegen test harness driver so that it no
longer modifies the environment GOOS/GOARCH, since that seems to cause
flakiness in other concurrently-running tests.

The change also enables the codegen tests in run.go.

Fixes #24538

Change-Id: I997ac1eb38eb92054efff67fe5c4d3cccc86412b
Reviewed-on: https://go-review.googlesource.com/103455


Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>