rustbuild: only plan from build triple for dist

We only want to package each host/target once for `dist`. The obvious
solution takes the form of step dependency, which is implemented at
least for the `dist-rustc` step. Unfortunately since the steps are
created from `hosts x targets` during planning and *not* de-duplicated
afterwards, the problem still persists.

We therefore move the check inside `plan()` instead, to avoid creating
the duplicate steps in the first place.

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