fix(frontend): use `NEXT_PUBLIC_AGPT_SERVER_URL` on proxy route (#10280)

### Changes 🏗️

A new undocumented env var, `NEXT_PUBLIC_AGPT_SERVER_BASE_URL`, was
added to the proxy route for it to work with the new `react-query`
mutator.

I removed it and used the existing `NEXT_PUBLIC_AGPT_SERVER_URL`, so we
have fewer environment variables to manage ( _and this one is already
added to all environments_ ).

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] Run the server locally
  - [x] All pages ( library, marketplace, builder, settings ) work