Package appimage with linuxdeploy

[mssalvatore]

What does this PR do?

Fixes #1355
Fixes #1362

Add any further explanations here.

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Testing Checklist

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  Tested by running on Ubuntu Bionic, Ubuntu Focal, and Rocky 8

[codecov]

Codecov Report

Merging #1381 (762d398) into develop (b4d97d1) will not change coverage.
The diff coverage is n/a.

❗ Current head 762d398 differs from pull request most recent head 1148836. Consider uploading reports for the commit 1148836 to get more accurate results

@@           Coverage Diff            @@
##           develop    #1381   +/-   ##
========================================
  Coverage    39.80%   39.80%           
========================================
  Files          466      466           
  Lines        13808    13808           
========================================
  Hits          5496     5496           
  Misses        8312     8312           

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[VakarisZ]

Why unlink instead or rm?

[mssalvatore]

Probably because some of them were symlinks, and I use unlink for symlinks instead of rm; not because it does anything substantively different, just because that's how my brain works. I can make them rm instead.