`process.env.*` vars should be read once at the top of the file

### What is this about?

> we may be doing it elsewhere but as a point of style I like to read the ENV at the beginning of the module, and use that state elsewhere

_Originally posted by @kumavis in https://github.com/MetaMask/metamask-extension/pull/22524#discussion_r1480352595_

No files or tests are named. Search the TypeScript sources for process.env.* references and review how each module currently reads them. Done means the relevant environment variables are read once at the top of each affected module and reused, with the existing test suite still passing.

Written by the indexing model from the issue text.