fix(nix): provide libclang in the dev shell

[AlexBSoD]

Problem

Building any nRF firmware from inside nix develop fails in the nrf-mpsl-sys build script:

error: failed to run custom build command for `nrf-mpsl-sys v0.2.1`

  Unable to find libclang: "couldn't find any valid shared libraries matching:
  ['libclang.so', 'libclang-*.so', 'libclang.so.*', 'libclang-*.so.*'], set the
  `LIBCLANG_PATH` environment variable to a path where one of these files can
  be found (invalid: [])"

nrf-mpsl-sys and nrf-sdc-sys generate their bindings with bindgen, which dlopens libclang at build-script time. The dev shell ships the full Rust toolchain, the embedded targets, flip-link, probe-rs, cargo-binutils — but no libclang, so anyone building firmware has to find a clang in the store and export LIBCLANG_PATH by hand every time.

Fix

Set LIBCLANG_PATH to pkgs.llvmPackages.libclang in the dev shell env.

Testing

Built keyboards/k04 central from a clean CARGO_TARGET_DIR inside nix develop, with no manually exported variables — nrf-mpsl-sys and nrf-sdc-sys both compile and the build finishes. alejandra --check is clean.

Note

No BINDGEN_EXTRA_CLANG_ARGS is added: a plain cargo build doesn't need it, and scripts/build_k04_matrix.sh already derives its own when it wants ARM headers.

[AlexBSoD]

Rebased and green where it can be: the Build workflow passes, and the red CI run is not from this change — check and the build (examples/use_rust/*) jobs fail the same way on main itself (runs 31499869616, 31484036006, 31393491006).

This branch only touches flake.nix, so it cannot affect those jobs. Happy to look into the main breakage separately if that helps.