chore: revert to pnpm@8 because turborepo doesn't support pnpm@9

Local dependencies weren't detected, so something as simple as `turbo build` would explode.

See vercel/turborepo#8168.

loader/package.json

@@ -57,7 +57,7 @@
     "fs-extra": "^11.2.0",
     "pino": "^8.21.0",
     "pino-pretty": "^11.2.1",
-    "pnpm": "^9.4.0",
+    "pnpm": "^8.15.8",
     "rimraf": "^5.0.7",
     "typescript": "^5.5.2"
   }

package.json

@@ -44,7 +44,7 @@
     "turbo": "^2.0.4",
     "typescript": "^5.5.2"
   },
-  "packageManager": "pnpm@9.4.0",
+  "packageManager": "pnpm@8.15.8",
   "pnpm": {
     "peerDependencyRules": {
       "allowedVersions": {