rustbuild: clarify comment on target array calculation

The comment touched, as originally written, only concerned itself with
the `test` steps. However, since #38468 the `arr` variable actually has
gained an indirect relationship with the `dist` steps too. The comment
failed to convey the extra meaning, contributing to the misunderstanding
which eventually lead to #38637. Fix that by moving the comment into the
right place near the relevant condition, and properly documenting
`arr`'s purpose.

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