bild: create out-link for Nix image builds

bild does not produce the out-link for Nix image builds. We have to run a separate nix-build to create _/nix/... symlinks.

Expected:

• bild Omni/Agentd/Images/Base.nix should create/update _ /nix/Omni/Agentd/Images/Base.nix out-link (like other bild outputs).

Actual:

• bild succeeds but no out-link is created, so docker load < _/nix/... fails unless we run nix-build ... --out-link _/nix/... manually.

Goal:

• Make bild create/update out-links for Nix image targets, or otherwise make bild + docker load < _/nix/... work without a manual nix-build step.

Notes:

• This affects agentd container images (Base/Git/Haskell).
• Prefer a general fix in bild rather than a one-off workaround.