A .deb image for cargo-tarpaulin would be very nice for speeding up CI. It's totally possible, since other cargo plugins like cargo-lichking exist. If you decide to take this seriously, talk to the Debian rust-maintainers mailing list to get started: https://qa.debian.org/developer.php?login=pkg-rust-maintainers%40alioth-lists.debian.net
Not having to rebuild wasmtime from scratch every time would also be nice, but, that's a bit tougher since it's a lib built into the wossname. Investigate how that might work.
Packaging Rust stuff for Debian is... well, not simple, but also pretty mature. Sources: https://blog.hackeriet.no/packaging-rust-part-II/ for info, https://salsa.debian.org/rust-team/debcargo-conf for authoritative tooling, https://wiki.debian.org/Teams/RustPackaging for team and contact info.
In the current state, adding cargo-tarpaulin to this would involve adding a couple libs and upgrading ~10ish others. The debian-rust team is quite friendly and responsive on IRC, and would be happy to have whatever work I do on this contributed back upstream.
Suggestions from them: "start with adding the new stuff, that will have to go through the new que for manual review by the ftp masters [for license compliance], and that might take some time"
Currently hacked together with a pre-built binary of tarpaulin, which knocks a cool 5 min off the CI time. But, that just means that building wasmtime for end-to-end unit tests now takes the majority of the time.