chore(ui): rename env var to datachain

[shcheklein]

Minor quick fix

[cloudflare-workers-and-pages]

Deploying datachain-documentation with    Cloudflare Pages

Latest commit:	d1d0c89
Status:	✅  Deploy successful!
Preview URL:	https://527d6122.datachain-documentation.pages.dev
Branch Preview URL:	https://chore-rename-dvc-datachain.datachain-documentation.pages.dev

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.30%. Comparing base (f79173a) to head (d1d0c89).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #779      +/-   ##
==========================================
- Coverage   87.33%   87.30%   -0.03%     
==========================================
  Files         128      128              
  Lines       11181    11181              
  Branches     1514     1514              
==========================================
- Hits         9765     9762       -3     
- Misses       1041     1043       +2     
- Partials      375      376       +1     

Flag	Coverage Δ
datachain	87.24% <ø> (-0.03%)	⬇️

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