[nix] Fix RocksDB building on non-linux

[georgeee]

In recent PR #17192 I accidentally broke rocksdb building on non-linux platforms (under nix).

This PR fixes it by ensuring that we attempt to use rocksdb 5.x.x only on linux-x86_64.

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

[glyh]

This should no longer be relevant