commit ensure-perms script for image post-processing

this was created weeks ago and not committed

scripts/ensure-perms

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+#!/usr/bin/env bash
+
+# ensures perms on a newly-built distribution are good.
+# usage: sudo ensure-perms /path/to/nix
+
+nix_path=$1
+chown root:root -R $nix_path
+chown root:nixbld $nix_path/store