app_store: use fixed vfs::open_dir

[nick1udwig]

Problem

A broken process_lib function led to some broken code here.

Solution

Use fixed process_lib code & fix code here.

Testing

Does it work? Note master should work as well

Docs Update

None

Notes

None

[dr-frmr]

Can we put out process_lib 0.9.1 and upgrade all usages in runtime to that new version?

[barraguda]

nice thank you! tested app_store downloads interactions with this, lgtm

[barraguda]

actually @nick1udwig , should we add open_or_create_dir() as a helper function to process_lib? seems like something we would use reasonably often.
Same with open_or_create_file()?

potential PR here: hyperware-ai/process_lib#92
can update usage in core too

edit: no new helpers needed, just a further adjustment in open_dir!