Fix issue #6934: explain how to use apt-system-requirements.txt

[mhucka]

Change the instructions involving "apt install" to use the file
apt-system-requirements.txt, which is the purpose of the file.

Resolves #6934

[pavoljuhas]

Please update the PR title so it is descriptive without having to lookup the issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.66%. Comparing base (4436b85) to head (36ff3bb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7230   +/-   ##
=======================================
  Coverage   98.66%   98.66%           
=======================================
  Files        1106     1106           
  Lines       95914    95914           
=======================================
  Hits        94630    94630           
  Misses       1284     1284           

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[mhucka]

Please update the PR title so it is descriptive without having to lookup the issue.

Haha, I just updated it because I thought the same thing, and then read your comment.