nixos/tests/prometheus: increase `memorySize`
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] affb72eccd5284defab1b557be65f45f8a02c86d

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs