github actions: minor updates to Windows CI

use a platform-specific name for build artifacts, and use VS2019 toolset instead of VS2017 toolset.

.github/workflows/main.yml

@@ -40,12 +40,12 @@ jobs:
       shell: cmd
       env:
         PLATFORM: ${{ matrix.platform }}
-      run: cmake -Bbuild -Hmegasource -T v141 -A %PLATFORM% -DCMAKE_INSTALL_PREFIX=%CD%\install
+      run: cmake -Bbuild -Hmegasource -T v142 -A %PLATFORM% -DCMAKE_INSTALL_PREFIX=%CD%\install
     - name: Install
       shell: cmd
       run: cmake --build build --config Release --target install
     - name: Artifact
       uses: actions/upload-artifact@v1
       with:
-        name: love
+        name: love-windows-${{ matrix.platform }}
         path: install