Fix Clippy error about pedantic group priority

[edmorley]

With newer versions of Rust, Clippy was failing on main with:

error: lint group `pedantic` has the same priority (0) as a lint
  --> Cargo.toml:23:1
   |
23 | pedantic = "warn"
   | ^^^^^^^^   ------ has an implicit priority of 0
24 | unwrap_used = "warn"
   | ----------- has the same priority as this lint
   |
   = note: the order of the lints in the table is ignored by Cargo
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#lint_groups_priority
   = note: `#[deny(clippy::lint_groups_priority)]` on by default
help: to have lints override the group set `pedantic` to a lower priority
   |
23 | pedantic = { level = "warn", priority = -1 }
   |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

eg:
https://github.com/heroku/buildpacks-apt/actions/runs/9955852891/job/27504486097?pr=32#step:5:188

See also:
heroku/buildpacks-python#184