Environment variables not available in `onPreDev` plugin event

[eduardoboucas]

Describe the bug

@biilmann has reported that he's not seeing the URL environment variable in the onPreDev event of a build plugin when running netlify dev --live. We should do a review of the environment variables we're exposing and ensure they're consistent with what's available to functions.

https://github.com/netlify/cli/pull/5607

Steps to reproduce

1. Create a build plugin with a onPreDev that prints process.env
2. Create a function that prints process.env
3. Run netlify dev --live
4. Compare the output of the two sources