Don't override finding executables when using pre-built tools.

Actually, we shouldn't really need this. We are building native: false binaries so when we look up a native: true binary the override should not apply. I've fixed this upstream in meson in https://github.com/NixOS/nixpkgs/pull/86080, though some backwards compatibility migration might be in order.

In the meantime, we can make gi_cross_use_host_gi prevent the overrides from happening, which will achieve the desired behavior.

Finally, this allows us to use find_program in scanner_command, unconditionally, letting the presence of the override dictate whether a freshly-built or pre-built g-ir-scanner is used.