Make `getToken` work on Cloudflare Workers by not eagerly evaluating defaults

[AlexErrant]

☕️ Reasoning

Cloudflare Workers don't define process.env. getToken defaults to process.env in an eager fashion, resulting in errors like "process is not defined". This PR uses ??= to set some params lazily.

🧢 Checklist

• [ ] Documentation
• [ ] Tests
• [x] Ready to be merged

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Name	Status	Preview	Comments	Updated (UTC)
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[ndom91]

This makes a lot of sense. Are you still interested in getting this merged? If so, please check out the merge conflict :pray:

[AlexErrant]

Looks like getToken on main no longer uses process.env