When installing a plugin, if it contains a `PulumiPlugin.yaml` file with a `runtime` value of `nodejs` or `python`, install dependencies for the plugin.
For Node.js, `npm install` is run (or `yarn install` if `PULUMI_PREFER_YARN` is set).
For Python, a virtual environment is created and deps installed into it.
We make several calls to `os/user`, which uses CGO and means
cross-compilation is not possible. This replaces `os/user` with the
`luser` package, which is a drop-in replacement which does not use `CGO`