get pub before downloading prebuild

Author: MrCyjaneK

.github/workflows/pr_test_build_windows.yml

@@ -75,6 +75,8 @@ jobs:
     
       - name: download monero_c prebuilds (can't build inside of windows container)
         run: |
+          cp pubspec_base.yaml pubspec.yaml
+          flutter pub get
           dart run tool/download_moneroc_prebuilds.dart
 
       - name: Execute Build and Setup Commands