genesis: use env to configure pre-genesis chain-id

[Fraccaman]

Describe your changes

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
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[tzemanovic]

the change lgtm, I'll just add some test that running a node, test-genesis and sign-genesis-txs are all picking this correctly

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.82%. Comparing base (be38d4c) to head (d269374).
Report is 27 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/node/src/utils.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3833   +/-   ##
=======================================
  Coverage   72.82%   72.82%           
=======================================
  Files         338      338           
  Lines      104220   104227    +7     
=======================================
+ Hits        75897    75903    +6     
- Misses      28323    28324    +1     

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