bug#40966: Missing substitutes on ci.guix.gnu.org?

[Leo Famulari]

I've noticed that certain packages never seem to have substitutes
available from ci.guix.gnu.org.

This is not about packages that consistently fail to build, like Vigra
(LibreOffice) [0], but packages that never seem to be attempted at all,
and are not found when searching on the CI results.

For example, ncmpcpp has not been built since February 2020:

https://ci.guix.gnu.org/search?query=spec%3Aguix-master+system%3Ax86_64-linux+ncmpcpp

And Borg has apparently never been built:

https://ci.guix.gnu.org/search?query=spec%3Aguix-master+system%3Ax86_64-linux+borg

By the way, the dates that are shown in this interface do not display
the year (can we fix that?):

https://ci.guix.gnu.org/build/2275489/details

[0] <https://bugs.gnu.org/40887>