Fix determining the current executable under Homebrew for Linux

We need to use `os.Lstat` instead of `os.Stat` if we are going to
manually inspect files for whether they are a symlink. On macOS, it
looks like `os.Stat` returns information about the symlink itself.

pkg/cmdutil/factory.go

@@ -68,7 +68,7 @@ func executable(fallbackName string) string {
 		if err != nil {
 			continue
 		}
-		f, err := os.Stat(p)
+		f, err := os.Lstat(p)
 		if err != nil {
 			continue
 		}

pkg/cmdutil/factory_test.go

@@ -0,0 +1,59 @@
+package cmdutil
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func Test_executable(t *testing.T) {
+	testExe, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testExeName := filepath.Base(testExe)
+
+	// Create 3 extra PATH entries that each contain an executable with the same name as the running test
+	// process. The first is a symlink, but to an unrelated executable, the second is a symlink to our test
+	// process and thus represents the result we want, and the third one is an unrelated executable.
+	dir := t.TempDir()
+	bin1 := filepath.Join(dir, "bin1")
+	bin1Exe := filepath.Join(bin1, testExeName)
+	bin2 := filepath.Join(dir, "bin2")
+	bin2Exe := filepath.Join(bin2, testExeName)
+	bin3 := filepath.Join(dir, "bin3")
+	bin3Exe := filepath.Join(bin3, testExeName)
+
+	if err := os.MkdirAll(bin1, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.MkdirAll(bin2, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.MkdirAll(bin3, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if f, err := os.OpenFile(bin3Exe, os.O_CREATE, 0755); err == nil {
+		f.Close()
+	} else {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(testExe, bin2Exe); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(bin3Exe, bin1Exe); err != nil {
+		t.Fatal(err)
+	}
+
+	oldPath := os.Getenv("PATH")
+	t.Cleanup(func() {
+		os.Setenv("PATH", oldPath)
+	})
+	os.Setenv("PATH", strings.Join([]string{bin1, bin2, bin3, oldPath}, string(os.PathListSeparator)))
+
+	if got := executable(""); got != bin2Exe {
+		t.Errorf("executable() = %q, want %q", got, bin2Exe)
+	}
+}