Check the deferred Close calls so the linter is happy

errcheck flagged six naked defer Close() calls in the pack code and its
tests. Wrap them in the same defer func() { _ = x.Close() }() form the rest
of the tree already uses.

cli/open.go

@@ -37,7 +37,7 @@ func runOpen(ctx context.Context, path, addr string, openBrowser bool) error {
 	if err != nil {
 		return fmt.Errorf("cannot open %q: %w", path, err)
 	}
-	defer r.Close()
+	defer func() { _ = r.Close() }()
 
 	ln, err := net.Listen("tcp", addr)
 	if err != nil {