fix: make the find_resource! macro responsible for the absolutize() call (#8884)

https://github.com/openai/codex/pull/8879 introduced the
`find_resource!` macro, but now that I am about to use it in more
places, I realize that it should take care of this normalization case
for callers.

Note the `use $crate::path_absolutize::Absolutize;` line is there so
that users of `find_resource!` do not have to explicitly include
`path-absolutize` to their own `Cargo.toml`.

codex-rs/utils/cargo-bin/Cargo.toml

@@ -9,4 +9,5 @@ workspace = true
 
 [dependencies]
 assert_cmd = { workspace = true }
+path-absolutize = { workspace = true }
 thiserror = { workspace = true }