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.