Install missing dotnet versions.

Author: dlemstra

.github/workflows/build-and-test.yml

@@ -90,6 +90,15 @@ jobs:
                   key: ${{ runner.os }}-nuget-${{ hashFiles('**/*.csproj', '**/*.props', '**/*.targets') }}
                   restore-keys: ${{ runner.os }}-nuget-
 
+            - name: DotNet Setup
+              uses: actions/setup-dotnet@v1
+              with:
+                dotnet-version: |
+                  6.0.x
+                  5.0.x
+                  3.1.x
+                  2.1.x
+
             - name: Build
               shell: pwsh
               run: ./ci-build.ps1 "${{matrix.options.framework}}"