S3 cache does not provide realisations endpoint for content addressed derivations

[gador]

Describe the bug

When I build a content addressed derivation with

nix store make-content-addressed nixpkgs#hello
nix copy --to "file://$PWD/temporary-cache?compression=zstd" /nix/store/ysk9lpkljlk6kf33rigng5hjhjvrll8i-hello-2.12.1
nix-store --delete /nix/store/ysk9lpkljlk6kf33rigng5hjhjvrll8i-hello-2.12.1
nix copy --from file://$PWD/temporary-cache /nix/store/ysk9lpkljlk6kf33rigng5hjhjvrll8i-hello-2.12.1

There is a realisations folder and I can copy the derivation from the temporary store.
However, if i use an S3 store with:

nix build --impure --expr '(import <nixpkgs> { config.contentAddressedByDefault = true; }).bash'
nix copy --to 's3://nix-cache?profile=nixbuilder&endpoint=s3.hel.lan&region=eu-central-1' /nix/store/vif5q2hs10x4cr6bb5fs8d8la9j1ygf0-bash-5.2p32
nix-store --delete /nix/store/vif5q2hs10x4cr6bb5fs8d8la9j1ygf0-bash-5.2p32
nix build --impure --expr '(import <nixpkgs> { config.contentAddressedByDefault = true; }).bash' --extra-substituters "https://s3.hel.lan/nix-cache/" -j0

it doesn't fetch bash and tries to build it. If I remove -j0 it builds bash but still does not use the cache.
All the builds are automatically signed by nix.settings.secret-key-files and the public key is used by nix.settings.trusted-public-keys

Also, there is no realisations folder in my s3 bucket.

Steps To Reproduce

See above

Expected behavior

There should be no need to rebuild bash but should be fetched from the s3 store

nix-env --version output

nix-env (Nix) 2.18.8

Additional context

AFAIK there is no cache software (nix-serve, nix-serve-ng, harmonia, attic) that provides the realisations endpoint. How does this work with hydra in light of NixOS/hydra#838 ? How does the test cache from https://discourse.nixos.org/t/content-addressed-nix-call-for-testers/12881 (https://cache.ngi0.nixos.org) get populated by ca-derivations?

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