internal/syscall/execenv: refactor handling env variables

internal/syscall/execenv: refactor handling env variables

Discover while working on CL 471335.

Change-Id: I006077a5aa93cafb7be47813ab0c4714bb00d774
Reviewed-on: https://go-review.googlesource.com/c/go/+/471435
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>