build: Don't treat warnings as errors in CI

[seb-cr]

Same problem here as we saw in react-donation: with the new Concourse Tools, the warnings that were previously ignored are being treated as errors, causing the build to fail.

Treating warnings as errors because process.env.CI = true.
Most CI servers set it automatically.

Failed to compile.

./src/components/FormHeader/FormHeader.js
src/components/FormHeader/FormHeader.js
  Line 18:6:  React Hook useEffect has a missing dependency: 'props.page'. Either include it or remove the dependency array. If 'setPage' needs the current value of 'props.page', you can also switch to useReducer instead of useState and read 'props.page' in the reducer  react-hooks/exhaustive-deps

...

asset size limit: The following asset(s) exceed the recommended size limit (244 KiB).
This can impact web performance.
Assets: 
  static/js/vendors.047a79c4.chunk.js (1.24 MiB)

entrypoint size limit: The following entrypoint(s) combined asset size exceeds the recommended limit (244 KiB). This can impact web performance.
Entrypoints:
  main (1.38 MiB)
      static/js/runtime~main.18f2596e.js
      static/js/vendors.047a79c4.chunk.js
      static/js/main.6d6b121f.chunk.js

See also https://github.com/comicrelief/react-donation/pull/1334

Relates to ENG-4087