CI: clippy job is not failing on documentation errors

[rcgoodfellow]

For example this error and many others like it are currently making it through CI.

warning: unresolved link to `StNestedBuilder::make_subsection`
   --> nexus/types/src/sectioned_table.rs:145:7
    |
145 | /// [`StNestedBuilder::make_subsection`].
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `StNestedBuilder` in scope
    |
    = note: `#[warn(rustdoc::broken_intra_doc_links)]` on by default

[iliana]

I figure what is missing in our clippy task is RUSTDOCFLAGS="-Dwarnings":

omicron/.github/buildomat/jobs/clippy.sh

Lines 30 to 33 in e30978a

	banner clippy
	export CARGO_INCREMENTAL=0
	ptime -m cargo xtask clippy
	ptime -m cargo doc

But while trying to test that theory, I found I can't reproduce those messages locally running cargo doc. Not sure why.