os/exec: gofmt

CL 109361 introduced some changes which were not properly gofmt'ed.
Because the CL was sent via Github no gofmt checks were performed
on it (cf. #24946, #18548).

Change-Id: I207065f01161044c420e272f4fd112e0a59be259
Reviewed-on: https://go-review.googlesource.com/115356


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>