go/packages: skip TestCompiledGoFilesIncludesDepsErrors without cgo

TestCompiledGoFilesIncludesDepsErrors uses cgo to
produce a dependency error. Plan 9 does not support
cgo, so skip it when cgo is unavailable, like the
other cgo tests.

Change-Id: I2e6e2c514cadbc0ace9778ad0e93a3d7b3f3abca
Reviewed-on: https://go-review.googlesource.com/c/tools/+/794842
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: 0intro

go/packages/packages_test.go

@@ -3573,6 +3573,7 @@ func main() {
 // See golang/go#78083.
 func TestCompiledGoFilesIncludesDepsErrors(t *testing.T) {
 	testenv.NeedsGoPackages(t)
+	testenv.NeedsTool(t, "cgo")
 
 	dir := writeTree(t, `
 -- go.mod --