Allow --validator-dir to be specified after subcommands (#8329)

#3768


  Made the --validator-dir flag global so that it can be specified in any order


Co-Authored-By: Mike Jerred <mjerred.work@gmail.com>

Co-Authored-By: chonghe <44791194+chong-he@users.noreply.github.com>

account_manager/src/validator/mod.rs

@@ -28,6 +28,7 @@ pub fn cli_app() -> Command {
                     "The path to search for validator directories. \
                     Defaults to ~/.lighthouse/{network}/validators",
                 )
+                .global(true)
                 .action(ArgAction::Set)
                 .conflicts_with("datadir"),
         )