Works around <https://issues.guix.gnu.org/55441#12>.
Start from commit bd86bbd300474204878e927f6cd3f0defa1662a5,
'open-inferior' uses 'primitive-fork' instead of 'open-pipe*'. As a
result, child process could potentially hang before calling 'execl' due
to undefined behavior when forking a multi-threaded process.
* build-aux/cuirass/evaluate.scm <top level>: Call 'open-inferior'
before 'n-par-for-each'.
This removes hydra support to use Cuirass as the only continuous integration
system.
* build-aux/hydra/gnu-system.scm: Remove it.
* build-aux/hydra/guix-modular.scm: Ditto.
* build-aux/hydra/guix.scm: Ditto.
* build-aux/cuirass/hydra-to-cuirass.scm: Ditto.
* Makefile.am (EXTRA_DIST): Update it.
(hydra-jobs.scm): Remove it.
(cuirass-jobs.scm): Update it.
* build-aux/hydra/evaluate.scm: Move it to ...
* build-aux/cuirass/evaluate.scm: ... here.
* build-aux/cuirass/guix-modular.scm: Remove it.
* build-aux/cuirass/gnu-system.scm: Ditto.
* guix/packages.scm (%hydra-supported-systems): Rename it to ...
(%cuirass-supported-systems): ... this variable.
* build-aux/check-final-inputs-self-contained: Adapt it.
* etc/release-manifest.scm: Ditto.
* gnu/ci.scm (package->alist): Remove it.
(derivation->job): New procedure.
(package-job, package-cross-job, cross-jobs, image-jobs, system-test-jobs,
tarball-jobs): Use it.
(guix-jobs): New procedure.
(hydra-jobs): Rename it to ...
(cuirass-jobs): ... this procedure.
