No llvm-tools-preview out of box in nix shell

[JSAbrahams]

Description of Bug

How to Reproduce

Steps to reproduce the behaviour, with steps and resulting error.

coverage-json
info: cargo-llvm-cov currently setting cfg(coverage); you can opt-out it by passing --no-cfg-coverage
error: failed to find llvm-tools-preview, please install llvm-tools-preview, or set LLVM_COV and LLVM_PROFDATA environment variables

Expected behavior

Additional context