build: add build tag noselfupdate

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

Fixes #5187

cmd/selfupdate_enabled.go

@@ -0,0 +1,7 @@
+// +build !noselfupdate
+
+package cmd
+
+// This constant must be in the `cmd` package rather than `cmd/selfupdate`
+// to prevent build failure due to dependency loop.
+const selfupdateEnabled = true