Updating workflow (dotnet#542)

Updating workflow for correct versions and variables:
- [x] Use v2 on checkout for faster perf
- [x] Use `dotnet-version` instead of deprecated property
- [x] Update to latest on setup-dotnet

.github/workflows/format.yml

@@ -12,12 +12,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2
 
     - name: Setup .NET Core SDK
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v1.5.0
       with:
-        version: 3.1.100
+        dotnet-version: 3.1.100
 
     - name: dotnet tool restore
       run: dotnet tool restore