Fix gen_localnet.py json exception (backport #4089)

[mergify]

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
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

---

This is an automatic backport of pull request #4089 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.58%. Comparing base (a3e1747) to head (cac1f85).

Additional details and impacted files

@@             Coverage Diff             @@
##           maint-0.46    #4103   +/-   ##
===========================================
  Coverage       74.58%   74.58%           
===========================================
  Files             341      341           
  Lines          107484   107484           
===========================================
+ Hits            80166    80172    +6     
+ Misses          27318    27312    -6     

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