cmd/dist: simplify exec.Cmd helpers
Now that the required Go bootstrap version is 1.17, we can simplify
some of the logic for working with exec.Cmd.Env as those APIs have
been simplified.
Updates #44505.
Change-Id: Ia62ab9830320c7daf65a15f2313471dfaba687b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/427957
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>