fix(maker-flatpak): add eu-strip to required external binaries

[malept]

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Summarize your changes:

Needed due to zypak sandboxing for Electron >= 5.

Corresponding docs update: electron-forge/electron-forge-docs@2aa0f59

Fixes #1702

[codecov]

Codecov Report

Merging #1703 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1703   +/-   ##
=======================================
  Coverage   70.26%   70.26%           
=======================================
  Files          74       74           
  Lines        2189     2189           
  Branches      412      412           
=======================================
  Hits         1538     1538           
  Misses        465      465           
  Partials      186      186           

Impacted Files	Coverage Δ
packages/maker/flatpak/src/MakerFlatpak.ts	86.95% <100.00%> (ø)

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