os/exec: adjust function name in fatal message

os/exec: adjust function name in fatal message

CL 481620 changed the test to use LookPath.

Change-Id: Ie7b3110775b586701b6df6aeee38f26402227eb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/529095
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Bypass: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>