`reuse` and `when_possible` concretization broke the invariant that
`spec[pkg_name]` has unique keys. This invariant is relied on in tons of
places, such as when setting up the build environment.
When using `when_possible` concretization, one may end up with two or
more `perl`s or `python`s among the transitive deps of a spec, because
concretization does not consider build-only deps of reusable specs.
Until the code base is fixed not to rely on this broken property of
`__getitem__`, we should disable reuse in CI.
b1559cc831