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/queue-runner/notifications.t

@@ -8,7 +8,7 @@ my $binarycachedir = File::Temp->newdir();
 
 my $ctx = test_context(
     nix_config => qq|
-    experimental-features = nix-command
+    experimental-features = nix-command ca-derivations
     substituters = file://${binarycachedir}?trusted=1
     |,
     hydra_config => q|