bug#41146: [Cuirass] Build non-substitutable packages

[Brice Waegeneire]

Hello Guix,

It would be useful to build non-substitutable packages on cuirass, 
without offering substitutes for it, to catch broken packages.  For 
example ATM the zfs package is broken but it hasn't been noticed because 
it isn't present on ci.guix.gnu.org[0].

This wish item follows a discussion on IRC[1].

[0]: https://ci.guix.gnu.org/search?query=zfs
[1]: http://logs.guix.gnu.org/guix/2020-05-09.log#102425

- Brice