build: pin the package manager the config belongs to

The gate runs `corepack enable; pnpm install`, and package.json named no
packageManager — so CI took whatever pnpm the runner happened to carry. That is
why declaring onlyBuiltDependencies twice changed nothing: the setting moved
between pnpm versions, and neither location matched the version actually
running.

Pinned to the one this was verified against. With it, locally:

    pnpm install --frozen-lockfile   Done in 2s, exit 0
    pnpm build                       built in 1.29s
    dist/icons.es.js                 IconInsightsQL x2, IconHogQL x0

An unpinned package manager is the same defect as an unpinned linter — the
thing that reads your config has to be the thing you tested the config with.

Co-authored-by: Hanzo Dev <[email protected]>

package.json

@@ -41,5 +41,6 @@
     "typescript": "^5.0.2",
     "vite": "^4.4.0",
     "vite-plugin-dts": "^3.2.0"
-  }
+  },
+  "packageManager": "[email protected]"
 }