pnpm won't build on less than node 18.12

[stuartf]

Fix for this error when building:

ERROR: This version of pnpm requires at least Node.js v18.12
The current version of Node.js is v18.8.0
Visit https://r.pnpm.io/comp to see the list of past pnpm versions with respective Node.js version support.
The command '/bin/sh -c pnpm i' returned a non-zero code: 1
ERROR: Service 'payload' failed to build : Build failed