os/exec: remove duplicate environment variables in Cmd.Start

os/exec: remove duplicate environment variables in Cmd.Start

Nobody intends to have duplicates anyway because it's so undefined
and everything handles it so poorly.

Removing duplicates automatically simplifies code and makes existing
code do what people already expect.

Fixes #12868

Change-Id: I95eeba8c59ff94d0f018012a6f4e031aaabfd5d9
Reviewed-on: https://go-review.googlesource.com/37586
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>