go/build: replace os.Setenv with T.Setenv
go/build: replace os.Setenv with T.Setenv

Updates #45448

Change-Id: I4d8e5d7e57818355ef2bc33b57ddf9c8b8da3e62
Reviewed-on: https://go-review.googlesource.com/c/go/+/310030
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Michael Knyszek <4473450dc5c88f06e1fb787f303d7805ec2aee65@google.com>

golang/go,cloudflare/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go