flora-6: configure more agressive garbage

collection

Reason: it has already happened a few times, that flora-6 ran out of
disk space. With this fix, hopefully the garbage collection should
kick in earlier and prevent this from happening

hosts/flora-6/flora-6.nix

@@ -54,6 +54,14 @@ in {
       }
     ];
 
+    # Override nix.conf for more agressive garbage collection
+    nix.extraOptions = lib.mkForce ''
+      min-free = 536870912
+      keep-outputs = false
+      keep-derivations = false
+      fallback = true
+    '';
+
     # Machine user for CI pipelines
     users.users.hakkonaut = {
       description = "CI and automation user";