backwards/manatee/tadpole: Clean /tmp on boot

Only `pinwheel` set this. Elsewhere files in `/tmp` survived reboots,
and leftover `nh` build result symlinks there kept acting as GC roots,
pinning store paths that should have been collected.

hosts/tadpole/modules/boot/default.nix

@@ -30,6 +30,8 @@ in
     };
 
     boot = {
+      tmp.cleanOnBoot = true;
+
       loader = {
         grub = {
           enable = true;