fix jemalloc in nix

[kpinter-iohk]

Description

• Upgrading nixpkgs recently broke tikv-jemalloc-sys, requiring a c flag to be turned on. (Thanks @gilligan)
• This PR also bumps fenix

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• New tests are added if needed and existing tests are updated.
• Relevant logging and metrics added
• CI passes. See note on CI.
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

Note on CI

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