docs: add missing PORTABLE_EXECUTABLE_FILE

[felipecrs]

This environment variable is set and it's very useful, but the documentation missed it.

• electron-builder/packages/app-builder-lib/templates/nsis/portable.nsi

  Line 78 in 8ffd9d4

  System::Call 'Kernel32::SetEnvironmentVariable(t, t)i ("PORTABLE_EXECUTABLE_FILE", "$EXEPATH").r0'

• Refs [Feature] Option: Add games as Non-Steam Game Heroic-Games-Launcher/HeroicGamesLauncher#1331
• Refs Get executable APP name in main.js for portable #3186

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