guix system: Compute 'bootcfg' only for 'init' and 'reconfigure'.

guix system: Compute 'bootcfg' only for 'init' and 'reconfigure'.

Previously, 'guix system vm' would start by computing the bootcfg
derivation, which itself depended on an incorrect OS derivation (for the
original OS instead of the one passed through
'virtualized-operating-system'.)  That added overhead and would force
the user's config file to define a root file system, for example, even
though it makes no sense in the case of a VM.

* guix/scripts/system.scm (perform-action)[bootcfg]: Limit to the 'init'
and 'reconfigure' actions.