cmd/go/internal/script: Fix build tags of isETXTBSY
syscall.ETXTBSY is only meaningful on Unix platforms, not on Windows.
Fixes #67356
Change-Id: I1b13292d821d61d820b396e08be9a4dd1683e843
Reviewed-on: https://go-review.googlesource.com/c/go/+/585695 Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>