bug#30760: guix system init broken on non GuixSD

[Tomáš Čech]

`guix system init` seems to be broken for non GuixSD distirbutions:
When I tried it on openSUSE:

# guix system --no-bootloader init /Devel/git/guix-config/config.scm /mnt/mnt/
;;; note: source file /Devel/extra/gnu/packages/connman.scm
;;;       newer than compiled /root/.config/guix/latest/gnu/packages/connman.go
;;; note: source file /Devel/extra/gnu/packages/connman.scm
;;;       newer than compiled 
/usr/lib64/guile/2.2/site-ccache/gnu/packages/connman.go
;;; note: source file /Devel/extra/gnu/packages/connman.scm
;;;       newer than compiled 
/usr/lib64/guile/2.2/site-ccache/gnu/packages/connman.go
guix system: error: open-file: No such file or directory: 
"/run/booted-system/kernel/lib/modules/4.15.6-1-default/modules.alias"

4.15.6-1-default is version of my running kernel, but not defined as package - 
it is not expected to be used for guix call.

/run/booted-system/ is specific for GuixSD.

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