Introduce mainnet/devnet build profiles in nix (rampup)

[georgeee]

Clone of #15026 against rampup.

To build mainnet node, use nix build mina#mainnet

For devnet node: nix build mina#devnet

Explain how you tested your changes:

• Ran a node, checked that mainnet profile was used when compiling
• Compared output of mina advanced constraint-system-digests between nix version and mainnet binary built by CI

Checklist:

• 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
• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? None

[georgeee]

!ci-build-me

[georgeee]

Not needed anymore as we are abandoning rampup