Fetch crates through importCargoLock with an accepted User-Agent

[thedavidmeister]

Refs rainlanguage/rainix#377. Replaces the approach in #24.

What

cargoDeps is importCargoLock again, called with a fetchurl that appends --user-agent Nixpkgs to curl. fetchCargoVendor and its hash are gone.

Why

#24 moved crate fetching to fetchCargoVendor, whose hash covers Cargo.lock. The automated release commit that followed (bce8de4, the crate version bump) changed one line of Cargo.lock and the build has failed on a hash mismatch since; every consumer that bumped its rain input today had to pin 91d70ac instead of main. importCargoLock needs no hash, so a version bump cannot break it. Its only problem was the User-Agent: crates.io answers 403 to anything starting with curl/, which is what nixpkgs fetchurl sends, and accepts any other value. Overriding the agent on the fetcher fixes the download; the crate tarballs are fixed-output derivations keyed by content hash, so every cache hit is unchanged.

QA

• Discriminating tests: n/a, packaging only; the proof is the build.
• Mutations applied: n/a, no logic changed.
• Oracle: nix build .#rain on this branch, on top of bce8de4, fetched all 762 crate tarballs and produced the binary; the fetch log for alloy-provider 1.8.3 shows the download completing from the API URL that returns 403 under the default agent. bce8de4 itself fails to build on main.
• Category check: rainix#377 asks for rain-cli rebuilds to stop 403ing on crate fetches without breaking on every release; covered.

After merge, consumers can point their rain input at main again.

🤖 Generated with Claude Code

https://claude.ai/code/session_01V8ViHcKLVk2YoS2joH4HdN

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