bug#38550: Cuirass: in job-name: replace 'version' with 'anchor'

["Clément Lassieur" <clement@lassieur.org>]

See IRC conversation[1].  The idea is that we want to be able to select
all the builds that have the same job-name.  Then we'll be able to see
the first one that's failing and its associated evaluation (the
culprit).

We could use PACKAGES->ANCHORS[2], which was created for the website.
Thus instead of having

 gajim-1.1.3.x86-64-linux <-fails gcc-7.4.x86_64-linux gcc-6.5.x86_64-linux
 gajim-1.1.3.x86-64-linux <-fails gcc-7.4.x86_64-linux gcc-6.5.x86_64-linux
 gajim-1.1.2.x86-64-linux <-fails gcc-7.3.x86_64-linux gcc-6.4.x86_64-linux
 gajim-1.1.2.x86-64-linux <-OK    gcc-7.3.x86_64-linux gcc-6.4.x86_64-linux

We'd have

 gajim.x86-64-linux <-fails       gcc.x86_64-linux     gcc-6.5.x86_64-linux
 gajim.x86-64-linux <-fails       gcc.x86_64-linux     gcc-6.5.x86_64-linux
 gajim.x86-64-linux <-fails       gcc.x86_64-linux     gcc-6.4.x86_64-linux
 gajim.x86-64-linux <-OK          gcc.x86_64-linux     gcc-6.4.x86_64-linux

And we could track the first failing Gajim (or GCC).

Each of the above column really represents a package definition tracked
in time.  GCC 7 and GCC 6 are two different packages at a given moment
in time, so they should have different job names.

Note that it doesn't work with packages for which we keep an old version
around if that version changes (e.g. from GCC 6.4 to GCC 6.5 in my
example).  But I imagine it's pretty rare.  Don't know.

This work is to be done in (gnu ci).

Comments welcome :)

Clément

[1]: http://logs.guix.gnu.org/guix/2019-12-09.log#171227
[2]: https://git.savannah.gnu.org/cgit/guix/guix-artwork.git/commit/?id=497145ef95cfc7548eb7c406d6227104f4b66700