Update build_and-test.yml

Fixes nbgv config.

.github/workflows/build_and-test.yml

@@ -11,6 +11,11 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0 # avoid shallow clone so nbgv can do its work.
+    - uses: dotnet/nbgv@v0.4.0
+      with:
+        setAllVars: true     
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with: