include custom env in devenv

now you can just meson devenv -C build and run whatever cargo commands in there
manually instead of needing a run_command for each

meson.build

@@ -52,6 +52,7 @@ endif
 cargo = find_program('cargo')
 cargo_env = environment()
 cargo_env.set('MESON_BUILD_ROOT', meson.project_build_root())
+meson.add_devenv(cargo_env)
 
 custom_target(
   'cargo-build',