Cargo wasn't passing the `--resource-suffix` to the final merge step,
breaking the filename for the search index in rust-lang/rust (and
probably many other use cases). In fact, Cargo wasn't passing the
RUSTDOCFLAGS at all. This commit changes Cargo to pass the same flags to
the final merge invocation as to each crate's rustdoc invocation. This
conflates flags for two somewhat different things, but we already share
the same flags across all crates so I think sharing them with the merge
makes sense.

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camelid deleted the fix-mergeable-doc-flags branch