oh, FFS: i think rust 1.89 broke this. the workflow run this morning used 1.88 and worked fine:
>   stable-x86_64-unknown-linux-gnu unchanged - rustc 1.88.0 (6b00bc388 2025-06-23)

and the one which broke in the afternoon used 1.89:
>   stable-x86_64-unknown-linux-gnu unchanged - rustc 1.89.0 (29483883e 2025-08-04)