add snaplink

I know snaplink should be put to the nix store, but currently I have no idea how to do it.

scripts/snaplink

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+_fatal() { >&2 echo "ERROR: $*"; exit 1; }
+mountpoint=$(zfs list -H -t filesystem -o mountpoint "$SANOID_TARGET")
+[[ -n "$mountpoint" ]] || _fatal "expected non-empty $SANOID_TARGET"
+snap_path="$mountpoint/.zfs/snapshot/$SANOID_SNAPNAME"
+[[ -d "$snap_path" ]] || _fatal "expected to find a directory in $snap_path"
+link_path="$mountpoint/.snapshot-latest"
+ln -sf "$snap_path" "$link_path"
+>&2 printf "successfully created %s -> %s\n" "$link_path" "$snap_path"