syscall: sort Windows env block in StartProcess

syscall: sort Windows env block in StartProcess

Fixes #29530

Change-Id: Ia28c78274b9288bfa5de9ccb142a452d291a5b66
Reviewed-on: https://go-review.googlesource.com/c/go/+/694435
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>