fix: pin pnpm for Node 18 deployments

nixpacks.toml

@@ -1,7 +1,7 @@
 [phases.setup]
 nixPkgs = ['nodejs_18', 'openssl']
 cmds = [
-  'npm install -g pnpm@latest',
+  'npm install -g pnpm@10.34.5',
   'npm install -g prisma'
 ]