ci: Make sure the Guix checkout is the one providing Cuirass proc.

ci: Make sure the Guix checkout is the one providing Cuirass proc.

Fixes <https://bugs.gnu.org/36378>.
Reported by Reza Alizadeh Majd <r.majd@pantherx.org>.

* build-aux/hydra/gnu-system.scm (find-current-checkout): New procedure.
(hydra-jobs): Use FIND-CURRENT-CHECKOUT to define CHECKOUT.  Pass it to the
inferior Guix as an extra argument whose key is 'superior-guix-checkout'.
* gnu/ci.scm (find-current-checkout): New procedure.
(hydra-jobs): Use FIND-CURRENT-CHECKOUT to define CHECKOUT.  This will return
'#f' if the current Guix is an inferior.  In that case, use the
'superior-guix-checkout' argument provided by the superior Guix.