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.
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