fix: add install phase with --no-frozen-lockfile for linux native bindings

rolldown requires platform-specific bindings (@rolldown/binding-linux-x64-gnu)
that are absent from a lockfile generated on a different OS.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nixpacks.toml

@@ -1,5 +1,8 @@
 [phases.setup]
 nixPkgs = ["nodejs_23", "pnpm"]
 
+[phases.install]
+cmds = ["pnpm install --no-frozen-lockfile"]
+
 [phases.build]
 cmds = ["pnpm build"]