When running `cargo doc -Zrustdoc-scrape-example`, it performs
additional type checks that a plain old `cargo doc` doesn't.
That leads to some extra failure when adopting scrape-example for docs.rs.
In de34d60 we introduced `unit_can_fail_for_docscraping` in order to
make `-Zrustdoc-scrape-example` not fail whenever `cargo doc` builds.
Build script executions were accidentally included in the list of
fallible units. A plain `cargo doc` does fail when a build script
fails. `-Zrustdoc-scrape-example` should follow that.
