Fix lint errors with Rust 1.77

[edmorley]

Fixes:

warning: field `0` is never read
  --> test-buildpacks/readonly-layer-files/src/main.rs:34:13
   |
34 |     IOError(std::io::Error),
   |     ------- ^^^^^^^^^^^^^^
   |     |
   |     field in this variant
   |
   = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
34 |     IOError(()),
   |             ~~

warning: field `0` is never read
  --> test-buildpacks/sbom/src/main.rs:65:13
   |
65 |     IOError(std::io::Error),
   |     ------- ^^^^^^^^^^^^^^
   |     |
   |     field in this variant
   |
   = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
65 |     IOError(()),
   |             ~~

warning: field `0` is never read
  --> test-buildpacks/store/src/main.rs:39:13
   |
39 |     IOError(std::io::Error),
   |     ------- ^^^^^^^^^^^^^^
   |     |
   |     field in this variant
   |
   = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
   |
39 |     IOError(()),
   |             ~~

warning: field `0` is never read
   --> libcnb/src/layer/tests.rs:959:13
    |
959 |     IoError(std::io::Error),
    |     ------- ^^^^^^^^^^^^^^
    |     |
    |     field in this variant
    |
    = note: `#[warn(dead_code)]` on by default
help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
    |
959 |     IoError(()),
    |             ~~

GUS-W-15350320.