Ignore embedded-io version in cargo-outdated job

The way that `postcard` depends on 2 different versions of `embedded-io`
(as of jamesmunns/postcard#123) apparently fools `cargo outdated`.

    Name                   Project  Compat  Latest  Kind    Platform
    ----                   -------  ------  ------  ----    --------
    postcard->embedded-io  0.4.0    ---     0.6.1   Normal  ---
    postcard->embedded-io  0.6.1    0.4.0   ---     Normal  ---

.github/workflows/ci.yml

@@ -75,4 +75,4 @@ jobs:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - uses: dtolnay/install@cargo-outdated
-      - run: cargo outdated --workspace --exit-code 1
+      - run: cargo outdated --workspace --ignore embedded-io --exit-code 1