Add troubleshooting tip for if systemd doesn't start with WSL

[RossSmyth]

I was having this problem, and this was suggest to me and it worked.
https://matrix.to/#/!apXxbtexmNGJaAryzM:nixos.org/$MR4egL2Tz6dIcnSLk-zRY14fRoj1NN2-7qc5sgJiW9A?via=nixos.org&via=matrix.org&via=beeper.com

I don't know the details of why it works, but it would be good to have this written down.

#527
microsoft/WSL#10050 (comment)

[antoineco]

Is this only happening on nixos-unstable? Do we have the slightest idea why? Is this because of a newer systemd version that requires cgroups v2?

Potentially related upstream change:

• systemd: 255.9 -> 256.2 NixOS/nixpkgs#307068

(I can only assume because NixOS 24.05 currently ships with systemd 255.9, which isn't causing issues with the default WSL2 config as far as I can tell.)

[RossSmyth]

@K900 might know?

[K900]

Yes, systemd 256 requires cgv2. Also, I tried making a dynamic warning for this, but could not find any way to log things to the console during early boot :(

[antoineco]

@K900 thanks!

For whoever is interested in knowing the background for this change, I stumbled upon this:

• AI: ramp up cgroup v1 deprecation systemd/systemd#30852

(Linked from Arch's wiki)

If a warning was to be added to the README, it would be useful to mention:

• the condition in which the problem occurs (systemd 256, nixpkgs-unstable)
• the reason for the occurrence (WSL2 has cgroup v1 enabled by default)

[nzbr]

Giving a reason in the docs and maybe a link here or to the upstream issue would be nice I think. Also this should maybe have its own heading instead of being mixed in with the general troubleshooting tips (it can stay on the same page though)