[DOC] Document setting environment variable

[pavithraes]

Fixes #759

Description

This pull request:

• Adds a note about setting environment variable
• Minor misc clean-up

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

Additional information

Merge after the conda-store-ui feature is merged.

[netlify]

✅ Deploy Preview for kaleidoscopic-dango-0cf31d canceled.

Name	Link
🔨 Latest commit	fb29e0f
🔍 Latest deploy log	https://app.netlify.com/sites/kaleidoscopic-dango-0cf31d/deploys/65e0c344ba66c70008591345

[nkaretnikov]

@pavithraes Thanks for documenting this! I've left some comments for your consideration.

Note: this feature is already in conda-store-server, but it's still in review in conda-store-ui. So I'm going to mark this PR as blocked until that one lands, see: conda-incubator/conda-store-ui#354.

[nkaretnikov]

Please move changes in this file into a separate PR. To me, these look unrelated to the environment variables change, which means they can be merged separately (and likely much faster as well).

[pavithraes]

Thanks for your review :)

I did mention in the PR description that it includes some minor, miscellaneous clean-up, so I thought this would be alright.

However, I do agree we should aim for cleaner+specific PRs. I'll revert these updates.

[pavithraes]

@nkaretnikov Thanks for your review, I have addressed all your comments. :)

Please take another look and approve it when you get a chance. Of course, even after approval, we'll only merge after https://github.com/conda-incubator/conda-store-ui/pull/354 is merged.

[nkaretnikov]

@pavithraes Thanks, LGTM! I've left two comments, which would be nice to fix, but those are not blockers. OK to merge this after the companion UI PR lands: conda-incubator/conda-store-ui#354