xtask: Fix `lint --scope=only-fuzz` trying and failing to `build_web()`. #878

Job	Run time
deny	43s
build (ubuntu, stable, locked, true)	10m 3s
build (windows, stable, locked, false)	16m 24s
incremental-mutants	0s
lint (stable)	2m 6s
build (macos, stable, locked, false)	8m 25s
lint (nightly)	2m 35s
build (ubuntu, nightly, locked, false)	8m 22s
miri	1m 27s
lint (beta)	2m 23s
build (ubuntu, beta, locked, false)	8m 42s
build (ubuntu, stable, latest, false)	9m 28s
build (ubuntu, stable, minimal, false)	10m 17s
no_std	48s
features	23m 27s
fuzz	14m 1s
build-web	0s
deploy-web	0s
	1h 59m 11s

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