runtime: add cgo guard for exit hooks test

runtime: add cgo guard for exit hooks test

Add an additional guard to ensure that we don't try to run the "-race"
variant of the exit hooks test when CGO is explicitly turned off via
CGO_ENABLED=0 (this fixes a failure on the no-cgo builder caused
by CL 354790).

Change-Id: I9dc7fbd71962e9a098916da69d9119a753f27116
Reviewed-on: https://go-review.googlesource.com/c/go/+/434935
Run-TryBot: Than McIntosh <thanm@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>