copilot: Fix config dir logic to support Flatpak environments (#32901)

Closes #30784

In github copilot we were not handling the config path correctly for
FLATPAK.

* Only tested on mac don't have access to other platform. But this
should work on other platform as well. It follows the similar pattern
seen in zed config path resolution.
- [x] Macos
- [ ] Linux
- [ ] Linux with Flatpak
- [ ] Windows

Release Notes:

- Fix copilot config detection for flatpack

crates/copilot/Cargo.toml

@@ -29,6 +29,7 @@ chrono.workspace = true
 client.workspace = true
 collections.workspace = true
 command_palette_hooks.workspace = true
+dirs.workspace = true
 fs.workspace = true
 futures.workspace = true
 gpui.workspace = true