add nix flake

[tg-x]

Describe your changes

add nix flake & instructions

Indicate on which release or other PRs this topic is based on

Checklist before merging to draft

• I have added a changelog
• Git history is in acceptable state

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.92%. Comparing base (5cc23b4) to head (34a67e9).
Report is 1474 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3429   +/-   ##
=======================================
  Coverage   53.92%   53.92%           
=======================================
  Files         317      317           
  Lines      107575   107575           
=======================================
+ Hits        58007    58008    +1     
+ Misses      49568    49567    -1     

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[sug0]

this requires maintenance, and afaik no one in the namada core team has interest in maintaining this flake