Disable autodetect, pin to ubuntu 20

[brad-richardson]

Description

1. Disable autodetect and iconv to reduce library conflicts from prebuilts
2. Pin build to Ubuntu 20.04 to resolve linking of libmvec/libm and libpthread when built on 22.04 then later used on 20.04. This is due to those libraries being moved around: https://developers.redhat.com/articles/2021/12/17/why-glibc-234-removed-libpthread

Issues Fixed or Closed

N/A - progress towards ffmpeg5 upgrade

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes to documentation)
• Repository update (changes to repository files)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components