fixing --check saying outputs not valid

[Dave Love]

I ran build --check on a new package and it complained that some output
of the derivation aren't valid.  Then I checked hwloc, which also has a
"lib" output and it has the same problem:

  guix build: error: build failed: some outputs of 
`/gnu/store/bsafscmmaw43ssb0lhnri3vdi0wi0chx-hwloc-1.11.7.drv' are not valid, 
so checking is not possible

With --verbosity=4, it looks as if that's due to

  |   |   path `/gnu/store/fsch2h9r8jp3c5ahs0b0mr8pr4yp4l3q-hwloc-1.11.7-lib' 
is required, but there is no substituter that can build it

How could I fix that?