Revert "make-disk-image: replace nixos-install by nix-env calls"

This reverts commit e9bf955fd6d5ffda218878ba561c93c6a28dfe8e. We use
nixos-install to ensure that make-disk-image produces the same result
as a regular installation (9802da517fe4da41d68516029b01c5c1f175b3e4)
and to reduce code duplication. If there is something broken in
nixos-install, it should be fixed there.

nixos/lib/make-disk-image.nix

···
     export NIX_STATE_DIR=$TMPDIR/state
     nix-store --load-db < ${closureInfo}/registration
 
-    mkdir -m 0755 -p "$root/etc"
-    touch "$root/etc/NIXOS"
-
-    echo "copying system..."
-    nix-env --store "$root" --substituters "auto?trusted=1" \
-        -p "$root/nix/var/nix/profiles/system" --set "${config.system.build.toplevel}" --quiet
-
-    echo "copying channel..."
-    mkdir -p "$root/nix/var/nix/profiles/per-user/root"
-    nix-env --store "$root" --substituters "auto?trusted=1" \
-        -p "$root/nix/var/nix/profiles/per-user/root/channels" --set "${channelSources}" --quiet
+    echo "running nixos-install..."
+    nixos-install --root $root --no-bootloader --no-root-passwd \
+      --system ${config.system.build.toplevel} --channel ${channelSources} --substituters ""
 
     echo "copying staging root to image..."
     cptofs -p ${optionalString (partitionTableType != "none") "-P ${rootPartition}"} -t ${fsType} -i $diskImage $root/* /