Update impure-shell.nix: use go 1.19

[georgeee]

Problem: One of transitive dependencies that are brought by libp2p (though not activated in runtime) is hardcoded to fail compiling at Go1.21. This makes compiling from impure shell impossible.

Solution: downgrade go's version to 1.19

Change is contained to nix develop mina#impure, no other flow or command is affected.

Tested that cd src/app/libp2p_helper && make works from impure shell.

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [ ] Modified the current draft of release notes with details on what is completed or incomplete within this project
• [x] Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• [x] Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• [x] All tests pass (CI will check this if you didn't)
• [x] Serialized types are in stable-versioned modules
• [x] Does this close issues? None