nixos/proxmox-image: Disable O_DIRECT to fix assert

Context summary:
'vma create' can't otherwise write to tmpfs such as /dev/shm.
This is important when used from non-nixos machines which may
have /build as tmpfs.

VMA is Proxmox's virtual machine image format that wraps QEMU images,
augmenting these with proxmox-specific configuration file.
proxmox-image.nix uses the VMA tool to create vma image files.
The VMA tool exists as a patchset ontop of QEMU.

VMA writes its output with open() and O_DIRECT flag.
O_DIRECT does not work on Linux tmpfs [1]. Thus:
$ vma create ~/output.vma ...  # works, assuming home isn't tmpfs.
$ vma create /dev/shm/output.vma ...  # fails since /dev/shm is tmpfs
Failure results in assert(*errp == NULL).

O_DIRECT is a cache performance hint.
But it currently blocks our usage of nixos-generate -f proxmox from
Non-NixOS hosts and Docker.

The patch here simply removes O_DIRECT:
vma-writer.c later performs memalign due to O_DIRECT, but this is
safe to do with or without O_DIRECT.
Ideally, this should be fixed in upstream Proxmox: Perhaps by falling
back to open without O_DIRECT.

Another attempt to fix this SIGABRT is [2], which writes the vma file
directory to $out/ folder -- however that may still be tmpfs mounted
which it is in our case.

[1] https://lore.kernel.org/lkml/45A29EC2.8020502@tmr.com/t/
[2] https://github.com/NixOS/nixpkgs/pull/224282

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