build: limit published files

[Jayllyz]

@fastify/env currently publish unneeded files (test, github workflows etc). -> https://publint.dev/@fastify/[email protected]

Checklist

• [X] run npm run test && npm run benchmark --if-present
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct