Install .NET runtime

Update the PR build to install the version of the .NET runtime used in
our unit test runs, specifically, 7.0.x. This wasn't previously needed,
so I can only guess that the build system image we were using came with
it preinstalled. It seems we can no longer count on that, so now we will
explicitly install it.

Without this our unit tests fail to run due to a lack of a runtime.

eng/pipelines/templates/build-pull-request.yml

@@ -24,6 +24,14 @@ jobs:
   - powershell: npm --version
     displayName: Check npm version
 
+  # Ensure the .NET runtime needed by our unit tests is installed.
+  - task: UseDotNet@2
+    displayName: Install .NET Runtime
+    inputs:
+      packageType: runtime
+      # This should match the target of our unit test projects.
+      version: 7.0.x
+
   # Allows for accessing the internal AzDO feed (vs-impl-internal) for project restore via Azure Artifacts Credential Provider.
   # See: https://github.com/microsoft/artifacts-credprovider#automatic-usage
   # YAML reference: https://docs.microsoft.com/azure/devops/pipelines/tasks/package/nuget-authenticate?view=azure-devops