Make cargo codegen work irrelevant of CWD (#2913)

What the title says.

### What

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested [demo.rerun.io](https://demo.rerun.io/pr/2913) (if
applicable)

- [PR Build Summary](https://build.rerun.io/pr/2913)
- [Docs
preview](https://rerun.io/preview/pr%3Acmc%2Fcargo_codegen_anywhere/docs)
- [Examples
preview](https://rerun.io/preview/pr%3Acmc%2Fcargo_codegen_anywhere/examples)

crates/re_types_builder/src/bin/build_re_types.rs

@@ -1,5 +1,7 @@
 //! Helper binary for running the codegen manually. Useful during development!
 
+use camino::Utf8Path;
+
 const DEFINITIONS_DIR_PATH: &str = "crates/re_types/definitions";
 const ENTRYPOINT_PATH: &str = "crates/re_types/definitions/rerun/archetypes.fbs";
 const CPP_OUTPUT_DIR_PATH: &str = "rerun_cpp/src/rerun";
@@ -25,17 +27,28 @@ fn main() {
         }
     }
 
+    let workspace_dir = Utf8Path::new(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .and_then(|p| p.parent())
+        .unwrap();
+
+    let definitions_dir_path = workspace_dir.join(DEFINITIONS_DIR_PATH);
+    let entrypoint_path = workspace_dir.join(ENTRYPOINT_PATH);
+    let cpp_output_dir_path = workspace_dir.join(CPP_OUTPUT_DIR_PATH);
+    let rust_output_dir_path = workspace_dir.join(RUST_OUTPUT_DIR_PATH);
+    let python_output_dir_path = workspace_dir.join(PYTHON_OUTPUT_DIR_PATH);
+
     re_log::info!("Running codegen…");
     let (objects, arrow_registry) =
-        re_types_builder::generate_lang_agnostic(DEFINITIONS_DIR_PATH, ENTRYPOINT_PATH);
+        re_types_builder::generate_lang_agnostic(definitions_dir_path, entrypoint_path);
 
     re_tracing::profile_scope!("Language-specific code-gen");
     join3(
-        || re_types_builder::generate_cpp_code(CPP_OUTPUT_DIR_PATH, &objects, &arrow_registry),
-        || re_types_builder::generate_rust_code(RUST_OUTPUT_DIR_PATH, &objects, &arrow_registry),
+        || re_types_builder::generate_cpp_code(cpp_output_dir_path, &objects, &arrow_registry),
+        || re_types_builder::generate_rust_code(rust_output_dir_path, &objects, &arrow_registry),
         || {
             re_types_builder::generate_python_code(
-                PYTHON_OUTPUT_DIR_PATH,
+                python_output_dir_path,
                 &objects,
                 &arrow_registry,
             );