`api::path::resource_dir` does not return correct path for AppImage bundle

[gabcoh]

Describe the bug
The resource directory for the appimage bundle should be relative to the APPDIR variable (something like /tmp/.mount-xxxx/), but the system wide resource dir is returned instead.

To Reproduce
Steps to reproduce the behavior:

1. Print the results of api::path::resource_dir when running the appimage bundle

Expected behavior
A path relative to APPDIR is returned.

Platform and Versions (please complete the following information):

OS: Linux
Node: 15.8.0
NPM: 6.14.11
Yarn: 1.22.10
Rustc: 1.50.0

Additional context
Add any other context about the problem here.

Stack Trace

[gabcoh]

This should be an easy fix. Just have to check if the APPDIR variable is set and if so return paths relative to it. I'm going to try and solve this one, and hopefully should have a PR soon.