* build-aux/hydra/gnu-system.scm (package->alist): Call
'derivation-file-name' on the result of 'package-derivation'.
* build-aux/hydra/guix.scm (package->alist): Likewise.
* build-aux/hydra/gnu-system.scm (%core-packages): New variable.
(%packages-to-cross-build): Make an alias for %CORE-PACKAGES.
(hydra-jobs): Check for the 'subset' argument, and honor it for values
"all" and "core".
* build-aux/hydra/gnu-system.scm (hydra-jobs): Pick all the `system'
keys in ARGUMENTS.
[cross-jobs]: Turn into a procedure, with a `system' parameter.
Build packages for all of SYSTEMS and %CROSS-TARGETS.