add cargo as dependency to nix dev shell

[quiode]

The rust-analyzer vscode extension needs to execute cargo metadata before being able to build anything. This PR adds this dependency to the nix shell for it to work.
The ideal solution would be to remove this dependency in rust-analyzer or use the locally built version, so help / hints to achieve that would be appreciated.

The problematic error:

rust-analyzer failed to load workspace: Failed to load the project at /rust/src/tools/rust-analyzer/Cargo.toml: Failed to read Cargo metadata from Cargo.toml file /rust/src/tools/rust-analyzer/Cargo.toml, None: Failed to run `cd "/rust/src/tools/rust-analyzer" && RUSTC_BOOTSTRAP="1" RUSTUP_AUTO_INSTALL="0" "cargo" "metadata" "--format-version" "1" "--manifest-path" "/rust/src/tools/rust-analyzer/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"`: No such file or directory (os error 2)

[rustbot]

r? @Mark-Simulacrum

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