fix: delete old /pkg when new one is unzipped to there

[dr-frmr]

Problem

When a userspace package is installed, we unzip it into a /pkg folder. On subsequent installs, this folder was being overwritten but not wiped, leading to old files living on unless explicitly deleted.

Solution

These two small changes to bootstrap (runtime) and app store (userspace) make it such that a /pkg folder is always wiped before a package is unzipped into it.

Testing

• Boot a node, observe success
• Install an app, delete some files from /pkg, install it again, observe the deletion

Docs Update

We should document this aspect of development better in general.