re-order mounting debian apt source file

[pdabelf5]

Proposed changes

Avoid certificate warning when building the Debian plus images. Move debian apt source file to be used when gpg keys are armoured.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.86%. Comparing base (491e68e) to head (55af6cc).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6223      +/-   ##
==========================================
+ Coverage   45.58%   45.86%   +0.27%     
==========================================
  Files         142      142              
  Lines       18961    18961              
==========================================
+ Hits         8644     8696      +52     
+ Misses       9868     9818      -50     
+ Partials      449      447       -2     

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