When package.json pins pnpm via `packageManager` or an exact
`devEngines.packageManager.version`, self-update the bootstrap up
front. The bootstrap's `pnpm store path` skips pnpm's auto-switch
(the `store` command sets `skipPackageManagerCheck = true`), so it
reports the bootstrap's `STORE_VERSION` while the user's actual
install runs under the pinned version and writes to a different
STORE_VERSION — breaking `cache: true` and `setup-node`'s
`cache: pnpm` on cold caches.
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