I needed to se file->udev-rule in my definition:
(define %probe-rs-udev-rules (file->udev-rule "69-probe-rs.rules" (local-file "../files/69-probe-rs.rules")))
This builds, puts the file in the store and links to it in the right place (/etc/udev/rules.d).
I now need to go back and see what happens with cargo embed!
edit: cargo flash and cargo embed now work as expected.