chore: include .net 5 for the build tools

.github/workflows/build.yml

@@ -106,7 +106,10 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: '6.0.x'
+          dotnet-version: '6.0.x'      
+      - uses: actions/setup-dotnet@v1
+        with:
+          dotnet-version: '5.0.x'
 
       - name: Nuget Cache
         uses: actions/cache@v2