sibling let bindings extending one variable via dynamic attrpaths are rejected

[wyattgill9]

Two sibling let bindings that extend the same variable through dynamic attribute paths are rejected as a redefinition.

$ fix eval --expr 'let x="a"; y="b"; c.${x}=1; c.${y}=2; in c'
error: variable 'c' already defined at 1:29
  1 | let x="a"; y="b"; c.${x}=1; c.${y}=2; in c
    |                             ^ near `c`

$ nix-instantiate --eval -E 'let x="a"; y="b"; c.${x}=1; c.${y}=2; in c'
{ a = 1; b = 2; }

Only the both-dynamic case fails. Every static or mixed combination is accepted and correct:

$ fix eval --expr 'let x="a"; c.${x}=1; c.b=2; in c'
{ a = 1; b = 2; }

Hit by pkgs/by-name/lo/local-ai/tests.nix in the pinned nixpkgs.

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fix 0.3.0 at 1f1a99cf, built -Doptimize=ReleaseSafe with Zig 0.16.0, darwin/aarch64. Reference implementation is Nix 2.34.8.