Action fails if nbgv is already installed

[AArnott]

The dotnet tool install step returns a non-zero exit code if the tool is already installed, so it fails on repeated runs or if run after dotnet tool install already installed nbgv.

We should make this more resilient.

Also, we should consider not installing the tool globally. This is useful if the agent the action runs on is shared or reused across runs. Is that possible? In Azure Pipelines there are "private agents" which make it important for tasks to not change machine-wide state. Is such the case here?