This fixes a bug where different Guix branches gave the same
derivations.

This calls were using its previous name 'job-name'.

This is done because 'set-load-path!' returns a undefined value.

Move 'with-directory-excursion' here.

Don't create it in 'srcdir' to please 'make distcheck'.

Move it to (cuirass database).

* bin/cuirass.in (build-packages): Display derivation and output file
names.

* tests/gnu-system.scm (hydra-jobs): Add hello subset.

bin/cuirass.in (%options, show-help): Add --subset option.
(evaluate): Add ARGS argument.
(main): Adjust accordingly.

* src/cuirass/job.scm <job-spec>: New record type.
(%make-job-spec, make-job-spec, job-spec-name, job-spec-proc)
(job-spec-metadata): New procedures.
* tests/gnu-system.scm (package-job): Rename to ...
(package-job-spec): ... this.  Use 'make-job-spec'.
(package-cross-job): Rename to ...
(package-cross-job-spec): ... this.  Use 'make-job-spec'.
(tarball-jobs): Rename to ...
(tarball-job-specs): ... this.  Use 'make-job-spec'.
(package->alist): Rename to ...
(package-metadata): ... this.  Adapt.
(package->job): Rename to ...
(package->jobspec): ... this.  Adapt.
(hydra-jobs): Adapt.

* bin/cuirass.in (evaluate): Move 'build-packages' call to ...
(main): ... here.

* bin/cuirass.in (evaluate): Add STORE argument.  Move 'open-connection'
call to ...
(main): ... here.

* bin/cuirass.in (show-help): Remove reference to unbound variable
'program-name'.  Turn PROG into a mandatory argument.

* src/cuirass/job.scm <job>: Add 'metadata' field.
(job-metadata, %make-job): New procedures.

* tests/gnu-system.scm: New file.
* bin/cuirass.in (evaluate, main): Use it.

* src/cuirass/job.scm: New file.
* Makefile.am (dist_pkgmodule_DATA): Add it.
* bin/cuirass.in (evaluate, build-packages): Use it.