Revert "nixos/qemu-vm: Disable cache for $NIX_DISK_IMAGE"
Revert "nixos/qemu-vm: Disable cache for $NIX_DISK_IMAGE"

This reverts commit 6353f580f90c0fdd2b418fa853a78ec508bda2a5.

Unfortunately cache=none doesn't work with all filesystem options.

Hydra tests error out with: file system may not support O_DIRECT

See http://hydra.nixos.org/build/30323625/

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,triton/triton,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs,triton/triton,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs