test: use Turborepo pack task for test isolation (#92575)
### What?
Eliminates the expensive temporary repo directory (`tmpRepoDir`) copy
during isolated test setup by leveraging a Turborepo `pack` task with
caching.
### Why?
The previous test isolation flow copied the entire `packages/` directory
to a temp location, mutated every `package.json` to rewrite workspace
dependency references, and ran `pnpm pack` sequentially for each
package. This added ~10s+ of overhead per test suite. With Turborepo
caching, repeated runs are 500ms (and can still be improved).
### How?
- Adds a `pack-for-isolated-tests` task to `turbo.json` (depends on
`build`, outputs `packed.tgz`)
- Adds a `pack-for-isolated-tests` script (`pnpm pack --out
./packed.tgz`) to every workspace package
- Simplifies `linkPackages` in `repo-setup.js` to scan for pre-built
tarballs instead of copying/rewriting/packing
- Updates `create-next-install.js` to run `pnpm turbo run pack`
(benefits from caching) and use package manager `overrides`
(pnpm/npm/yarn) to resolve transitive workspace deps from local tarballs
- Removes `tmpRepoDir` handling from `base.ts` and `run-tests.js`
**Before:** `createNextInstall` ~16s (copy + sequential pack)
**After:** `createNextInstall` ~5s (first run) / ~4s (cached)
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