CI: better mimic docs.rs

[RickyDaMa]

See:

fontheight/.github/workflows/docs.yml

Lines 9 to 19 in 4575013

	# Make sure CI fails on all warnings, and pretend we're docs.rs
	env:
	RUSTFLAGS: "-Dwarnings -Adead-code -Aunused-imports -Aunused-macros"
	# FIXME: in future, use the below to better match docs.rs. At the time of
	# writing (2025/10/30), a very common documentation-related nightly
	# feature attribute (doc_auto_cfg) has been renamed (to doc_cfg) which
	# is resulting in the pipeline failing due to transient dependencies
	# using the removed feature (even though I specify --no-deps). Not
	# fully matching docs.rs also causes some warnings to get emitted
	# while building SLWL, so allow those for now.
	# RUSTFLAGS: "-Dwarnings --cfg docsrs"

A third party issue instead of us currently basically