Minimal CA support

This verison has a worse UI, but also chnages the schema less: One
non-null constraint is removed, but no new columns are added.

Co-Authored-By: Andrea Ciceri <andrea.ciceri@autistici.org>
Co-Authored-By: regnat <rg@regnat.ovh>

t/jobs/dir-with-file-builder.sh

@@ -0,0 +1,7 @@
+#! /bin/sh
+
+# Workaround for https://github.com/NixOS/nix/pull/6051
+echo "some output"
+
+mkdir $out
+echo foo > $out/a-file