Setup .NET Core

- Set dotnet-version to 2.2.108

Author: lahirumaramba

.github/workflows/nightly.yml

@@ -25,7 +25,7 @@ on:
 jobs:
   nightly:
 
-    runs-on: windows-2019
+    runs-on: windows-latest
 
     env:
       DOTNET_SKIP_FIRST_TIME_EXPERIENCE: 1
@@ -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: 2.2.108
 
     - name: Build with dotnet
       run: dotnet build FirebaseAdmin/FirebaseAdmin