flake: add `apps` entry and fix runtime error (#412)

* flake: add `apps` entry and fix runtime error

flake: fix package creation

* flake: update program path for default app

Co-authored-by: Steve Mathew Joy <45141270+RaySlash@users.noreply.github.com>

* flake: move rio package creation to external file

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Co-authored-by: Steve Mathew Joy <45141270+RaySlash@users.noreply.github.com>