CI: fix rust-nightly static checks

[portersrc]

This PR attempts to fix these two rust nightly errors we started seeing:

    Checking image-rs v0.1.0 (/home/porter/wo/confidential-containers/guest-components/image-rs)
error: trait `Protocol` is never used
  --> image-rs/src/resource/mod.rs:33:7
   |
33 | trait Protocol: Send + Sync {
   |       ^^^^^^^^
   |
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

error: direct implementation of `ToString`
  --> image-rs/src/signature/image/digest.rs:79:1
   |
79 | / impl ToString for Digest {
80 | |     fn to_string(&self) -> String {
81 | |         format!("{}:{}", self.algorithm, self.value)
82 | |     }
83 | | }
   | |_^
   |
   = help: prefer implementing `Display` instead
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#to_str
ing_trait_impl
   = note: `-D clippy::to-string-trait-impl` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::to_string_trait_impl)]`

[fitzthum]

Nice. The dead code warning is slightly interesting. I don't think that is dead code, but it's being consumed by another crate. I have a vague memory of this being a problem in the past. I'm not against the fix here.