ci: Fix unused deps (#36707)

Seen in
https://buildkite.com/materialize/nightly/builds/16530#019e5783-3714-4448-a887-684eeb57b1ab

Author: def-

src/mysql-util/Cargo.toml

@@ -51,3 +51,9 @@ mz-mysql-util = { path = ".", features = ["proptest"] }
 proptest.workspace = true
 proptest-derive.workspace = true
 tokio.workspace = true
+
+[package.metadata.cargo-udeps.ignore]
+# The self-dep above activates the `proptest` feature for tests so that
+# `cfg(test)`-gated `Arbitrary` derives can find `mz-repr`'s proptest impls.
+# cargo-udeps doesn't recognize feature-activation as usage.
+development = ["mz-mysql-util"]