Artemis Tosini (NixOS): since GitHub is broken at this time again, I'll bug you with review here :)
PR: https://github.com/NixOS/nix/pull/15793

tests/functional/dyn-drv/non-trivial.nix:
Are we phasing out $out for outputting derivations?
I'm leaning "good riddance" because this method had no purpose beyond a temporary stop gap for singleton derivations or perhaps small static graphs using multi-output derivations.
No need to hold on to that unnecessary code path.
If not, this should probably be a new test case instead of clobbering the old one.

Inside buildCommand:
Test that $out is unset?

src/libstore/unix/build/derivation-builder.cc/submitOutput:

• Refer to https://github.com/NixOS/nix/issues/12727 in the error message when not Opaque?
• Would be good to show the value of the previously submitted output
• Needs tests for both error scenarios

        curl_easy_setopt(req, CURLOPT_TCP_KEEPALIVE, 1L);
        curl_easy_setopt(req, CURLOPT_TCP_KEEPIDLE, 30L);
        curl_easy_setopt(req, CURLOPT_TCP_KEEPINTVL, 30L);
        curl_easy_setopt(req, CURLOPT_MAXAGE_CONN, 90L);

how about this. We may have been hitting a 60s timeout on the server side.. when matching ours there is a small race condition. Looks like 30s is fairly commonly used to prevent that.