Find nicer fix for utf-8-validate / bufferutil dep workaround

[hackergrrl]

291af8ae78bc952dd897e0a095cc553cbca364fd in mapeo-core adds the dependency websocket-stream, which depends on ws, which has dev dependencies utf-8-validate and bufferutil. However, ws has require() calls for these deps in its main code, wrapped in try/catch statements, which noderify doesn't seem to know to not include. This makes the backend build fail on mapeo-mobile. ws folx don't seem to want to change this.

For the time being, I've added --filter switches to the noderify call in scripts/build-backend.sh to ignore these two modules, but it's only a stopgap, since we may one day depend on a module which in turn depends on one of these for legitimate purposes.