Rename libp2p_helper in go.nix

[georgeee]

Move libp2p helper from bin/libp2p_helper to bin/mina-libp2p_helper in generated derivation. This allows importing derivation as buildInput without use of environment variable (not used in this PR, but will be used in future work).

Affects only nix, changes effectively nothing in operation of it, just updates some internals.

Explain how you tested your changes:

• Built Mina with nix, all good

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [x] 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

[georgeee]

!ci-build-me

[georgeee]

!ci-build-me