deps: Fix problematic serde by pinning to <= 1.0.171 (#546)

See: serde-rs/serde#2538

Apparently, `serde_derive` has been shipping a [non-reproducible
binary](serde-rs/serde#2538 (comment)).

The maintainer has also given [vague

reasons](serde-rs/serde#2538 (comment))
for why they want the newer releases to use a precompiled binary.
Supposedly, it helps with build time, but IMO that can't be a good
enough reason to ship a precompiled binary with.

Let's pin this until they decide to provide more clarity, since the
future of the project/library seems uncertain.