Don't use tmpfs for small-mem hosts

lib/fudo/hosts.nix

@@ -89,6 +89,13 @@ let
         default = null;
       };
 
+      tmp-on-tmpfs = mkOption {
+        type = bool;
+        description =
+          "Use tmpfs for /tmp. Great if you've got enough (>16G) RAM.";
+        default = true;
+      };
+
       enable-gui = mkEnableOption "Install desktop GUI software.";
 
       docker-server = mkEnableOption "Enable Docker on the current host.";
@@ -172,6 +179,8 @@ in {
       autoPrune.enable = true;
     };
 
+    boot.tmpOnTmpfs = host-cfg.tmp-on-tmpfs;
+
     programs.ssh.knownHosts = let
       keyed-hosts =
         filterAttrs (host: opts: opts.ssh-pubkey != null) config.fudo.hosts;