Fix Flutter Linux build by reordering CI steps

Move flutter config --enable-linux-desktop before flutter pub get to ensure Linux desktop support is enabled when dependencies are installed. Also add flutter precache --linux to download required Linux artifacts before building.

This fixes the CMake error about missing generated_plugins.cmake file by ensuring the Flutter build system properly generates all ephemeral files for Linux desktop.

Co-authored-by: jwinarske <[email protected]>

Author: Copilot

.github/workflows/build.yml

@@ -66,11 +66,14 @@ jobs:
           sudo apt-get update
           sudo apt-get install -y clang cmake ninja-build pkg-config libgtk-3-dev
       
+      - name: Enable Linux desktop
+        run: flutter config --enable-linux-desktop
+      
       - name: Install dependencies
         run: flutter pub get
 
-      - name: Enable Linux desktop
-        run: flutter config --enable-linux-desktop
+      - name: Precache Linux artifacts
+        run: flutter precache --linux
 
       - name: Build Linux release
         run: flutter build linux --release