os: skip TestExecutableDeleted earlier when 'go build' is not supported

os: skip TestExecutableDeleted earlier when 'go build' is not supported

The test unconditionally calls testenv.GoToolPath, which will skip the
test anyway. Moving the skip earlier gets this test out of goroutine
dumps if the test process fails or times out, making it easier to
diagnose failures in the remaining tests.

Change-Id: Ibd39546708a83b6f15616b2c4ae7af420e2401f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/475455
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>