revise move_to_app method in postflight mini-DSL

* use `@command.run` instead of `system_command` which is intended as an external interface
* quoting

Here I considered always setting multiple variants of `:key` to simplify Casks, but the each of the alternative keys only appear once in the repo.

Todo: as with all of these, it makes sense for this to be exposed symmetrically in `preflight` as well as `postflight`.

cc @federicobond