nix: mount tmpfs on /tmp

This hopefully eliminates spurious test failures caused by /tmp running out of space.

Signed-off-by: Ophestra <cat@gensokyo.uk>

test/configuration.nix

@@ -95,6 +95,9 @@
     ];
   };
 
+  # Disk image is too small for some tests:
+  boot.tmp.useTmpfs = true;
+
   environment.hakurei = {
     enable = true;
     stateDir = "/var/lib/hakurei";