CI: missing building of cargo-fuzz targets

[koivunej]

Looking at the existing examples in serde/json for example, there probably is currently no supported way around installing cargo-fuzz on a nightly and just cargo fuzz build the targets under each crates/interledger-*/fuzz.

Implementing this might be difficult for the caching, because these fuzz targets are not part of the workspace and have their own target/ directory, which might not work out of the box with the rust-cache action introduced in #732.

Originally posted by @koivunej in #732 (comment)