Bump pnpm from 8.15.4 to 9.15.0 in /.nix

Bumps [pnpm](https://github.com/pnpm/pnpm/tree/HEAD/pnpm) from 8.15.4 to 9.15.0.
- [Release notes](https://github.com/pnpm/pnpm/releases)
- [Changelog](https://github.com/pnpm/pnpm/blob/v9.15.0/pnpm/CHANGELOG.md)
- [Commits](https://github.com/pnpm/pnpm/commits/v9.15.0/pnpm)

---
updated-dependencies:
- dependency-name: pnpm
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

.nix/package.json

@@ -2,6 +2,6 @@
   "name": "mypackage",
   "version": "1.0.0",
   "dependencies": {
-    "pnpm": "8.15.4"
+    "pnpm": "9.15.0"
   }
 }