build action for `system' packages (like system.fix) that have
dependencies on all packages we want to activate.
So the command sequence to switch to a new activation configuration
of the system would be:
$ fix -i .../fixdescriptors/system.fix
...
system.fix -> 89cf4713b37cc66989304abeb9ea189f
$ nix-switch 89cf4713b37cc66989304abeb9ea189f
* A nix-profile.sh script that can be included in .bashrc.