build.rs: Locate binary in order of specificity

Philip Johanssonrequested to merge
progel/llvm-sys.rs:build.rs-binary-version into main

On my system both llvm-config and llvm-config-18 are installed. When I try to depend on llvm-sys with feature=["llvm18-1"], by default it uses llvm-config which is from version 22.

Instead of searching for llvm-config first, it should be the fallback binary.

Merge request reports