os/exec: fix GOOS=windows,Cmd.Run always calls LookPath before returning

Follow up on CL 511458, see https://go-review.googlesource.com/c/go/+/511458/2..4/src/cmd/go/main.go#b270 .

Fixes #36768

Change-Id: Icc2a4dbb1219b1d69dd10a900478957b0e975847

src/os/exec/exec.go

@@ -597,6 +597,15 @@ func lookExtensions(path, dir string) (string, error) {
 	if filepath.Base(path) == path {
 		path = "." + string(filepath.Separator) + path
 	}
+	exts := pathExt()
+	if ext := filepath.Ext(path); ext != "" {
+		for _, e := range exts {
+			if strings.EqualFold(ext, e) {
+				// Assume that path has already been resolved.
+				return path, nil
+			}
+		}
+	}
 	if dir == "" {
 		return LookPath(path)
 	}

src/os/exec/lp_plan9.go

@@ -64,3 +64,7 @@ func LookPath(file string) (string, error) {
 	}
 	return "", &Error{file, ErrNotFound}
 }
+
+func pathExt() []string {
+	return nil
+}

src/os/exec/lp_unix.go

@@ -80,3 +80,7 @@ func LookPath(file string) (string, error) {
 	}
 	return "", &Error{file, ErrNotFound}
 }
+
+func pathExt() []string {
+	return nil
+}

src/os/exec/lp_wasm.go

@@ -21,3 +21,7 @@ func LookPath(file string) (string, error) {
 	// Wasm can not execute processes, so act as if there are no executables at all.
 	return "", &Error{file, ErrNotFound}
 }
+
+func pathExt() []string {
+	return nil
+}

src/os/exec/lp_windows.go

@@ -63,6 +63,11 @@ func findExecutable(file string, exts []string) (string, error) {
 // As of Go 1.19, LookPath will instead return that path along with an error satisfying
 // errors.Is(err, ErrDot). See the package documentation for more details.
 func LookPath(file string) (string, error) {
+	return lookPath(file, pathExt())
+
+}
+
+func pathExt() []string {
 	var exts []string
 	x := os.Getenv(`PATHEXT`)
 	if x != "" {
@@ -78,7 +83,11 @@ func LookPath(file string) (string, error) {
 	} else {
 		exts = []string{".com", ".exe", ".bat", ".cmd"}
 	}
+	return exts
+}
 
+// lookPath implements LookPath for the given PATHEXT list.
+func lookPath(file string, exts []string) (string, error) {
 	if strings.ContainsAny(file, `:\/`) {
 		f, err := findExecutable(file, exts)
 		if err == nil {