Incorrect check for env constant breaks test cases.

[blackening]

https://github.com/source-academy/cadet-frontend/blob/c93fb65f26159d702e012632347cfc01f752b777/src/utils/constants.ts#L8-L10

Should be:

process.env.REACT_APP_USE_BACKEND &&
  process.env.REACT_APP_USE_BACKEND.toUpperCase() === 'TRUE';

The types for process.env are as follows:

export interface ProcessEnv {
    [key: string]: string | undefined;
}