nodejs: upgrade to pnpm 8+

librarian's Node.js toolchain currently pins `pnpm` to version `7.32.2`.

In `pnpm` v7, global binary directory resolution relies on `NPM_CONFIG_*` environment variables (`NPM_CONFIG_GLOBAL_BIN_DIR`, `NPM_CONFIG_STORE_DIR`, etc.) to prevent global CLI tools from leaking into the host's active Node.js installation directory.

Modern `pnpm` (v8+) standardized configuration exclusively around `PNPM_HOME` and `PNPM_CONFIG_*` environment variables.

### Proposed Changes
1. Upgrade `pnpm` in Librarian's Node.js generator toolchain to version `8.x` or later.
2. Clean up legacy `NPM_CONFIG_*` environment variables in `internal/librarian/nodejs/install.go` and rely exclusively on `PNPM_HOME` / `PNPM_CONFIG_*`.
3. Revert PR #6873

Start with internal/librarian/nodejs/install.go and trace the pinned pnpm version and environment variables used by the Node.js generator. Review PR #6873 and compare the current setup with the requested PNPM_HOME/PNPM_CONFIG_* configuration. Done means pnpm is upgraded to 8 or later, legacy NPM_CONFIG_* handling is removed, and global tools no longer affect the host Node.js installation.

Written by the indexing model from the issue text.