The e2e test harness gave each test its own store dir but left the
package-metadata cache at pnpm's shared default location. The mock
registry serves fixture packuments without cache-control headers, so once
one test cached a packument, a later test that changed that package's
dist-tag via addDistTag could resolve a stale version from the shared
cache. Set pnpm_config_cache_dir next to the per-test project dir so the
metadata cache is isolated too.
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