bug#36855: bug#37596: 'guix system switch-generation' does not reload Shepherd services

[Brice Waegeneire]

Hello Guix,

To summarise the whole thread, in addition to the main issue explained
in the title “'guix system switch-generation' does not reload Shepherd
services“ there are 2 other issues talked about.

1. The activation script isn't executed by 'guix system
 switch-generation', which is fixed in
 df138dc20858725b90ed77be85f3318cbe1be73a and later.
2. The activation process isn't exclusivly build during the system
 building process, whihc may the leave the system in half-hazard state
 if the building taking place during the activation fail.

The second side issue should proablby have it's own bug report since
it's still present and we should keep that bug report only for the lack
of reloading of shepherd service when using 'guix system
switch-generation' (or 'roll-back').

Cheers,
- Brice