cmd/go: fix cgo-disabled test failure from CL 77151

Change-Id: I8ed4c6dd32a74d94565d77771bb773f006ff90ac
Reviewed-on: https://go-review.googlesource.com/78317
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/go_test.go

@@ -2440,7 +2440,9 @@ func TestCoverageErrorLine(t *testing.T) {
 
 	tg.runFail("test", "coverbad")
 	tg.grepStderr(`coverbad[\\/]p\.go:4`, "did not find coverbad/p.go:4")
-	tg.grepStderr(`coverbad[\\/]p1\.go:6`, "did not find coverbad/p1.go:6")
+	if canCgo {
+		tg.grepStderr(`coverbad[\\/]p1\.go:6`, "did not find coverbad/p1.go:6")
+	}
 	tg.grepStderrNot(regexp.QuoteMeta(tg.tempdir), "found temporary directory in error")
 	stderr := tg.getStderr()