CI: Configure .NET SDK 8.0.100 to test min supported version

This is the current LTS and our minimum supported version,
so make sure that it builds fine.

.github/workflows/linux_builds.yml

@@ -132,6 +132,15 @@ jobs:
         if: '!matrix.legacy-scons'
         uses: ./.github/actions/godot-deps
 
+      - name: Setup .NET SDK
+        if: matrix.build-mono
+        uses: actions/setup-dotnet@v4
+        with:
+          # Targeting the oldest version we want to support to ensure it still builds.
+          dotnet-version: '8.0.100'
+        run: |
+          echo '{"sdk":{"version": "${{ steps.stepid.outputs.dotnet-version }}"}}' > ./global.json
+
       - name: Setup Python and SCons (legacy versions)
         if: matrix.legacy-scons
         uses: ./.github/actions/godot-deps
@@ -163,6 +172,7 @@ jobs:
       - name: Build .NET solutions
         if: matrix.build-mono
         run: |
+          dotnet --info
           ./modules/mono/build_scripts/build_assemblies.py --godot-output-dir=./bin --godot-platform=linuxbsd
 
       - name: Prepare artifact