nixos/tests/prometheus: increase memorySize

Apparently, our NixOS tests need more RAM now. This was needed for e.g.
the `gitea`-test as well[1].

[1] affb72e

nixos/tests/prometheus.nix

@@ -36,6 +36,7 @@ in import ./make-test-python.nix {
   nodes = {
     prometheus = { pkgs, ... }: {
       virtualisation.diskSize = 2 * 1024;
+      virtualisation.memorySize = 2048;
       environment.systemPackages = [ pkgs.jq ];
       networking.firewall.allowedTCPPorts = [ grpcPort ];
       services.prometheus = {
@@ -132,6 +133,7 @@ in import ./make-test-python.nix {
 
     store = { pkgs, ... }: {
       virtualisation.diskSize = 2 * 1024;
+      virtualisation.memorySize = 2048;
       environment.systemPackages = with pkgs; [ jq thanos ];
       services.thanos.store = {
         enable = true;