Don't hardcode the version (#353)

and add a `version` command that only prints the version (without the
build time)

Signed-off-by: Dimitris Karakasilis <dimitris@karakasilis.me>

main.go

@@ -2,10 +2,6 @@ package main
 
 import "github.com/mudler/luet/cmd"
 
-var (
-	version string
-)
-
 func main() {
 	cmd.Execute()
 }