Set .NET core version in Nightly builds

GHA `windows-latest` now use Windows 2022, which does not support `.net 2.1`. Setting the version in the workflow to see if that fixes it.

Author: lahirumaramba

.github/workflows/nightly.yml

@@ -36,6 +36,11 @@ jobs:
       uses: actions/checkout@v2
       with:
         ref: ${{ github.event.client_payload.ref || github.ref }}
+        
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 2.2.x
 
     - name: Build with dotnet
       run: dotnet build FirebaseAdmin/FirebaseAdmin