When using extlinux-conf-builder in a nix build using chroots, the
following error message could be seen:
/nix/store/XXX-extlinux-conf-builder.sh: line 121: cd: /nix/var/nix/profiles: No such file or directory
To avoid this, just skip the code path parsing /nix/var/nix/profiles
when $numGenerations (passed from the command line) is 0 (which is the
only legal value of $numGenerations in a nix build context).
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