Nix modules: change docspell user to be a normal user

Seems that unoconv requires a shell.

nix/buildvm.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+
+if [ "$1" = "-f" ]; then
+    echo "Deleting current state image..."
+    rm *.qcow2
+fi
+
+nixos-rebuild build-vm \
+  -I nixos-config=./configuration-test.nix \
+  -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.09.tar.gz