The attached patch makes isl build reproducibly in guix by disabling
parallelism.

Admittedly, I'm unsure what the root cause is, and Debian's build of isl
does not appear to have this problem, despite building in parallel.

Seems like patches to isl should go to core-updates:

  $ guix refresh --list-dependent isl                                                               
  Building the following 2192 packages would ensure 5967 dependent packages are rebuilt: ...


live well,
  vagrant