nixos/stage-1: Account for hardcoded executable paths
nixos/stage-1: Account for hardcoded executable paths

At least pkgs/os-specific/linux/util-linux/default.nix uses
```
    "--enable-fs-paths-default=/run/wrappers/bin:/run/current-system/sw/bin:/sbin"
```

which does not cover stage 1 init's PATH as all executables are put
under /bin/.

Fix util-linux's `mount` usage by symlinking /sbin to it.

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