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To reproduce issue on commit ba246100ca3b0c3bd0d68548677d75e6790c4f60
it does not suffice to add just `check-build-compiletest` to
`check-secondary`; one must also ensure that `check-build-compiletest`
precedes the satisification of the `check` rule.
Otherwise hidden dependencies of `compiletest` would end up getting
satisfied when make builds `rustc` at each stage in order to
eventually run `check-stage2`.
So to handle that I moved `check-secondary` before `check` in the
`check-all` rule that bors uses, and for good measure, I also put
`check-build-compiltest` at the front of the `check-secondary` rule's
dependencies.
My understanding is that running `check-secondary` should be
relatively cheap, and thus such a reordering will not hurt bors.
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Fix#17883.
b26972e28b