Fix llvm-lit path for LLVM 23 apt packages (#3620)
The `llvm-X-tools` apt package previously shipped `lit` as a Python
script at `build/utils/lit/lit.py`.
This path was removed in the Debian/Ubuntu packaging of LLVM 23:
["Install a lit
binary"](https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/commit/bedcb23bc00c9fe407725803d73e50ec6c8ecaf9)
The packaging now installs a compiled `lit` binary at `bin/lit` instead,
with a `/usr/bin/lit-X` symlink.
Updated LLVM_EXTERNAL_LIT` to match the new location.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/3c31709bed67c66
