CI: Install MinGW via `egor-tensin/setup-mingw`

Scoop install is being problematic since their 0.1.0 release.

(cherry picked from commit 3846201fbbb021380fb1b67e88cbfe00dc6c1dc3)

.github/workflows/ci.yml

@@ -90,13 +90,11 @@ jobs:
         run: |
           python -m pip install scons
 
+      - name: Setup MinGW for Windows/MinGW build
+        uses: egor-tensin/setup-mingw@v2
+
       - name: Build godot-cpp
-        # Install GCC from Scoop as the default supplied GCC doesn't work ("Error 1").
         run: |
-          Invoke-Expression (New-Object System.Net.WebClient).DownloadString('https://get.scoop.sh')
-          scoop install gcc
-          g++ --version
-          gcc --version
           scons target=release generate_bindings=yes use_mingw=yes -j $env:NUMBER_OF_PROCESSORS
 
       - name: Upload artifact