Use default pnpm package import method (#98703)
### What?
Use pnpm's default package import method in the two hoisted
isolated-test configurations added by #98425.
### Why?
`package-import-method` controls how regular package files are
materialized from pnpm's store: by reflink, hardlink, or copy. Those
choices do not affect `realpath`, so forcing copies is unrelated to the
Node.js symlink-resolution workaround and unnecessarily disables pnpm's
more efficient defaults.
### How?
Keep `node-linker=hoisted`, which is the setting responsible for
producing npm-style real package directories, while removing the
independent copy policy. The Node-version gate, local-tarball
validation, release-age policy, and fixture behavior remain unchanged.
### Verification
- Hoisted-only scratch install produced real package directories, no
package symlinks outside `.bin`, and a metadata-only `.pnpm`
- Production Turbopack: non-hoisted SWC helper and warm-restart task
stats passed (2/2)
- `pnpm build`
- `pnpm types`
- Prettier, ESLint, and `git diff --check`
Follow-up to #98425 and
https://github.com/vercel/next.js/pull/98425#discussion_r4020168942.
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Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
