Avoid spurious rebuilds of JSON docs in bootstrap

[Kobzol]

Found this while working on #159671.

Before, repeated runs of e.g. x dist rust-docs-json always rebuilt the docs. It was caused by Cargo not knowing that the output should be JSON, so it was trying to find the HTML files.

Instead of passing the output format through RUSTDOCFLAGS, which Cargo doesn't inspect, we now pass it to Cargo directly, which fixes the issue.

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