build: add build tag noselfupdate

Allow downstream packaging to build rclone without selfupdate command:
$ go build -tags noselfupdate

Fixes #5187

cmd/selfupdate/noselfupdate.go

@@ -0,0 +1,11 @@
+// +build noselfupdate
+
+package selfupdate
+
+import (
+	"github.com/rclone/rclone/lib/buildinfo"
+)
+
+func init() {
+	buildinfo.Tags = append(buildinfo.Tags, "noselfupdate")
+}