fix: set basePath from next config as env variable (#15154)

### What
Fix image preview URLs not displaying when `basePath` is set in
`next.config.mjs`.

### Why
When `basePath` is set in `next.config.mjs`, the `withPayload` function
assigns it as an env variable. However, currently the file sets
`basePath` as `baseConfig.env.NEXT_BASE_PATH`, which is only accessible
in Next.js runtime contexts (API routes, server components).

Upload URLs are generated from `formatAdminURL` in the Node.js
environment, where `baseConfig.env.NEXT_BASE_PATH` cannot be accessed.
As a result, it thinks no `basePath` is set and uses the incorrect URL
to render the upload preview.

### Where
`withPayload` now sets both `baseConfig.env.NEXT_BASE_PATH` **and**
`process.env.NEXT_BASE_PATH`, making `basePath` available everywhere.

Previously, users had to manually set `NEXT_BASE_PATH` in `.env` to work
around this.

##### Fixes #15111