Fix Linux desktop CI build by installing GTK3 dev dependency

[Copilot]

The desktop-build (ubuntu-latest, linux) Actions job was failing during flutter build linux because CMake could not find gtk+-3.0 on the Ubuntu runner. This updates the CI workflow to provision the required Linux desktop dependency only for the Linux matrix target.

• Root cause addressed

  • flutter build linux depends on GTK3 development headers via pkg-config.
  • Ubuntu runner image no longer guarantees libgtk-3-dev preinstalled.

• Workflow change (surgical)

  • Added a Linux-only install step in .github/workflows/ci.yml within desktop-build:
    • Runs apt-get update
    • Installs libgtk-3-dev
  • Scoped with if: matrix.target == 'linux' so Windows/macOS jobs are unaffected.

• Resulting CI behavior

  • Linux desktop build job now has the required GTK toolchain before flutter build linux.
  • No changes to app code or non-Linux CI paths.

- run: sudo apt-get update && sudo apt-get install -y libgtk-3-dev
  if: matrix.target == 'linux'

[Copilot]

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