octorust should consider enabling profile-hint-mostly-unused by default (once stable)

[soooch]

For context:

https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#profile-hint-mostly-unused-option

rust-lang/cargo#15644