Fix cid serde feature reference

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Somehow this was not caught in the cargo check, build, or lint but is caught when running with cargo run -p forest. Might be worth adding cargo c -p forest to the CI since it works when compiling workspace as a whole because the serde feature gets compiled since the IPLD crate defines the serde_derive feature on Cid but the forest binary does not currently connect with IPLD yet.
• Made serde_derive feature config more explicit on the serde implementations in Cid

Edit: Added cargo check -p forest to the CI so this doesn't happen again

Reference issue to close (if applicable)

Closes

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