make update-checksums use a temp dir for the cellar by default

cmd/bindown/config.go

@@ -44,14 +44,30 @@ type configUpdateChecksumsCmd struct {
 	TargetFile string `kong:"required=true,arg,help=${config_checksums_bin_help}"`
 }
 
-func (d *configUpdateChecksumsCmd) Run(*kong.Context) error {
+func (d *configUpdateChecksumsCmd) Run(kctx *kong.Context) error {
 	config, err := bindown.LoadConfigFile(cli.Configfile)
 	if err != nil {
 		return fmt.Errorf("error loading config from %q", cli.Configfile)
 	}
+	tmpDir, err := ioutil.TempDir("", "bindown")
+	if err != nil {
+		return err
+	}
+	defer func() {
+		err = os.RemoveAll(tmpDir)
+		if err != nil {
+			kctx.Errorf("error deleting temp directory, %q", tmpDir)
+		}
+	}()
+
 	binary := path.Base(d.TargetFile)
 	binDir := path.Dir(d.TargetFile)
 
+	cellarDir := cli.CellarDir
+	if cellarDir == "" {
+		cellarDir = filepath.Join(tmpDir, "cellar")
+	}
+
 	downloaders, ok := config.Downloaders[binary]
 	if !ok {
 		return fmt.Errorf("nothing configured for %q", binary)
@@ -60,7 +76,7 @@ func (d *configUpdateChecksumsCmd) Run(*kong.Context) error {
 	for _, downloader := range downloaders {
 		err = downloader.UpdateChecksum(bindown.UpdateChecksumOpts{
 			DownloaderName: binary,
-			CellarDir:      cli.CellarDir,
+			CellarDir:      cellarDir,
 			TargetDir:      binDir,
 		})
 		if err != nil {