Update on "[build] Fix flatc"

Fixes #8784

We need to install `build/pip_data_bin_init.py.in` into `<executorch
root>/data/bin/__init__.py`. This PR rewrite the logic into a
`BuiltFile` so that it works well in editable mode.

Since `BuiltFile` by default looks into cmake cache directory, this PR adds a placeholder `%CMAKE_CACHE_DIR%` for those are actually built by CMake and for `build/pip_data_bin_init.py.in` we don't add this placeholder.

Test:

```
python -c "from executorch.data.bin import flatc"
```
Will add unit test in next PR.

[ghstack-poisoned]

Author: larryliu0820

setup.py

@@ -360,7 +360,7 @@ def __init__(self, src: str, modpath: str):
             "/" not in modpath
         ), f"modpath must be a dotted python module path: saw '{modpath}'"
         # This is a real extension, so use the modpath as the name.
-        super().__init__(src=src, dst=modpath, name=modpath)
+        super().__init__(src=f"%CMAKE_CACHE_DIR%/{src}", dst=modpath, name=modpath)
 
     def src_path(self, installer: "InstallerBuildExt") -> Path:
         """Returns the path to the source file, resolving globs.