fix: allow using virtiofs share as writable /nix/store overlay

[songpola]

Fix #43

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The overlayfs already supported virtiofs since this PR: torvalds/linux@c6b80eb

Specifically, these two commits:

1. torvalds/linux@bccece1
2. torvalds/linux@d80172c

Now, looking at the current implementation (Linux Kernel v6.17), the overlayfs will check for xattr support of the upper , by trying to set (trusted|user).overlay.* xattr. By default, it will set using the trusted namespace, which requires CAP_SYS_ADMIN capability:

1. Which will fail the ovl_setxattr() call
2. noxattr will be set to true
3. And finally, this check will fail, since the virtiofs was considered a remote filesystem.

To mitigate this, we can use userxattr mount option to use the user namespace instead.

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Also, using the NixOS' new overlay fileSystems API can increase code readability and maintainability.

References:
NixOS/nixpkgs#290730
https://github.com/NixOS/nixpkgs/blob/nixos-unstable/nixos/modules/tasks/filesystems/overlayfs.nix

[SuperSandro2000]

The first 3 commits should be one as their are just fixups and the last commit should have the message from the 2nd last.

[songpola]

The first 3 commits should be one as their are just fixups and the last commit should have the message from the 2nd last.

Done. Please check.