Move nbgv installation in azp outside the src dir

The Agent.ToolsDirectory is a popular place to put versioned tools like this.

.gitignore

@@ -342,3 +342,6 @@ healthchecksdb
 
 # Backup folder for Package Reference Convert tool in Visual Studio 2017
 MigrationBackup/
+
+# dotnet tool local install directory
+.store/

azure-pipelines/build.yml

@@ -8,10 +8,9 @@ jobs:
   - template: install-dependencies.yml
 
   - powershell: |
-      dotnet tool install --tool-path .. nbgv --ignore-failed-sources
-      ../nbgv cloud
+      dotnet tool install --tool-path $(Agent.ToolsDirectory) nbgv --ignore-failed-sources
+      $(Agent.ToolsDirectory)/nbgv cloud
     displayName: Set build number
-    workingDirectory: src
 
   - template: dotnet.yml
   - template: expand-template.yml