execabs: make safe for Go 1.19

To preserve the same errors that LookPath used to return,
LookPath needs to know to ignore the new Go 1.19 exec.ErrDot errors.

For golang/go#43724.

Change-Id: I8938813a1a4e14bf697f05879e3ab212611644da
Reviewed-on: https://go-review.googlesource.com/c/sys/+/403256
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

execabs/execabs.go

@@ -53,7 +53,7 @@ func relError(file, path string) error {
 // LookPath instead returns an error.
 func LookPath(file string) (string, error) {
 	path, err := exec.LookPath(file)
-	if err != nil {
+	if err != nil && !isGo119ErrDot(err) {
 		return "", err
 	}
 	if filepath.Base(file) == file && !filepath.IsAbs(path) {