if a PKG is added as a target, always install it (#20971)

previously we were only installing on updates

fixes a bug found by @PezHub

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

<!-- Note that API documentation changes are now addressed by the
product design team. -->

- [x] Manual QA for all new/changed functionality
- For Orbit and Fleet Desktop changes:
- [x] Orbit runs on macOS, Linux and Windows. Check if the orbit
feature/bugfix should only apply to one platform (`runtime.GOOS`).
- [ ] Manual QA must be performed in the three main OSs, macOS, Windows
and Linux.
- [ ] Auto-update manual QA, from released version of component to new
version (see [tools/tuf/test](../tools/tuf/test/README.md)).

orbit/pkg/update/update.go

@@ -385,9 +385,8 @@ func (u *Updater) get(target string) (*LocalTarget, error) {
 				}
 			}
 			if strings.HasSuffix(localTarget.Path, ".pkg") && runtime.GOOS == "darwin" {
-				cmd := exec.Command("installer", "-pkg", localTarget.Path, "-target", "/")
-				if out, err := cmd.CombinedOutput(); err != nil {
-					return nil, fmt.Errorf("running pkgutil to install %s: %s: %w", localTarget.Path, string(out), err)
+				if err := installPKG(localTarget.Path); err != nil {
+					return nil, fmt.Errorf("updating pkg: %w", err)
 				}
 			}
 		} else {
@@ -398,6 +397,11 @@ func (u *Updater) get(target string) (*LocalTarget, error) {
 		if err := u.download(target, repoPath, localTarget.Path, localTarget.Info.CustomCheckExec); err != nil {
 			return nil, fmt.Errorf("download %q: %w", repoPath, err)
 		}
+		if strings.HasSuffix(localTarget.Path, ".pkg") && runtime.GOOS == "darwin" {
+			if err := installPKG(localTarget.Path); err != nil {
+				return nil, fmt.Errorf("installing pkg for the first time: %w", err)
+			}
+		}
 	default:
 		return nil, fmt.Errorf("stat %q: %w", localTarget.Path, err)
 	}
@@ -647,6 +651,14 @@ func extractTarGz(path string) error {
 	}
 }
 
+func installPKG(path string) error {
+	cmd := exec.Command("installer", "-pkg", path, "-target", "/")
+	if out, err := cmd.CombinedOutput(); err != nil {
+		return fmt.Errorf("running pkgutil to install %s: %s: %w", path, string(out), err)
+	}
+	return nil
+}
+
 func (u *Updater) initializeDirectories() error {
 	for _, dir := range []string{
 		filepath.Join(u.opt.RootDirectory, binDir),