build(flatpak): add xvfb locally

[ReenigneArcher]

Description

xvfb was removed from flatpak's Builder BaseApp in flathub/org.flatpak.Builder.BaseApp@402cd76

Screenshot

Issues Fixed or Closed

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)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

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

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 8.91%. Comparing base (9b5a046) to head (34ac47c).

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #2797      +/-   ##
=========================================
- Coverage    8.95%   8.91%   -0.04%     
=========================================
  Files          95      95              
  Lines       17386   17379       -7     
  Branches     8268    8268              
=========================================
- Hits         1557    1550       -7     
- Misses      12961   12964       +3     
+ Partials     2868    2865       -3     

Flag	Coverage Δ
Linux	6.78% <ø> (ø)
Windows	4.16% <ø> (ø)
macOS-12	?
macOS-13	9.93% <ø> (+0.01%)	⬆️
macOS-14	10.22% <ø> (ø)

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see 5 files with indirect coverage changes