On macOS 26.4, AFL builds were picking up Nix compiler-wrapper
variables and Apple SDK target settings from the shell environment.
That caused afl-cc to drive the wrong linker and target configuration,
which broke even simple fuzz harness builds. Unset the Nix compiler and
linker environment in the fuzz dev shell so AFL++ uses the system or
Homebrew Apple toolchain directly.
Also force afl-cc to link with lld because the newer Apple linker
asserts on the custom sections emitted by AFL's LLVM
instrumentation. Finally, pin fuzz-libghostty to the host target so the
build does not inherit stray SDK targets from the environment.
