Allow pass `registry` option to `get-pnpm`

[anytinz]

Currently, a constant is being used to pass the registry to get-pnpm:

setup/src/install-pnpm/download.ts

Line 57 in fbda4c8

await downloadVerifiedPnpm({ versionSpec: resolved.version, registry: REGISTRY, dest: destDir })

I’d be happy to create a PR for this. I have two ideas:

1. get-pnpm provides a registryFromEnv function that can be used to replace 'https://registry.npmjs.org'. This would align with the get-pnpm documentation, allowing users to pass the registry used for downloading pnpm via the npm_config_registry environment variable. We might also need to add this information to the project’s documentation.
2. Allow passing a registry as an input parameter.

We can implement this feature either by one of these approaches, or by combining both.