pnpm/action-setup bootstraps itself via npm before pnpm is available.
If a repo has a custom registry in .npmrc (e.g. pointing to JFrog or
Nexus), the self-installer tries to fetch pnpm from that registry
without credentials and fails with exit code 1.
Setting NPM_CONFIG_REGISTRY overrides .npmrc for this step only,
ensuring pnpm is always fetched from the public registry. Private
registry auth is configured in subsequent steps once pnpm is ready.
