loader: const fixes

loader: const fixes

Make the 'source_override' parameter of loader_get_manifest_files() const.
The const was removed in 74d013a5, seemingly for convenience of calling
loader_free_getenv(), not because it was incorrect.

Give loader_free_getenv() the same signature in all implementations.

Fix a memory leak where it was possible to exit loader_get_manifest_files()
without calling loader_free_getenv().

Don't pass a potentially NULL value to loader_log().