Enable using an erofs filesystem as one of the filesystems needed to
boot the system. This is useful for example in image based deployments
where the Nix store is mounted read only.
[erofs](https://docs.kernel.org/filesystems/erofs.html) offers multiple
benefits over older filesystems like squashfs. Skip fsck.erofs because
it is still experimental.
Calling `eval-config.nix` without a `system` from a Nix flake fails with
`error: attribute 'currentSystem' missing` since #230523. Setting
`system = null` removes the use of `currentSystem` and instead uses the
value from the `nixpkgs` module.
This speeds up evaluation by a factor 2.
Ballpark figures from my machine:
```
$ time nix-build nixos/release.nix -A tests.acme
/nix/store/q4fxp55k64clcarsx8xc8f6s10szlfvz-vm-test-run-acme
/nix/store/lnfqg051sxx05hclva84bcbnjfc71c8x-vm-test-run-acme
real 1m28.142s
user 1m7.474s
sys 0m7.932s
$ time nix-build nixos/release.nix -A tests.acme
/nix/store/q4fxp55k64clcarsx8xc8f6s10szlfvz-vm-test-run-acme
/nix/store/lnfqg051sxx05hclva84bcbnjfc71c8x-vm-test-run-acme
real 0m38.235s
user 0m33.814s
sys 0m2.283s
```
For this round of ZHF: #230712
Failing Hydra build: https://hydra.nixos.org/build/219234565
Not sure why this a problem now and not in the past, but routes to
the corresponding `/24`-subnet are only configured if addresses are
specified with the correct CIDR.
Fail pattern:
1. Unsuspecting `qemu-kvm` notice:
```
server # qemu-kvm: at most 2047 MB RAM can be simulated
```
2. Hard fail
```
self.shell.send(out_command.encode())
BrokenPipeError: [Errno 32] Broken pipe
```
(Took me a while to consider those lines are related)
Fail pattern:
1. Unsuspecting `qemu-kvm` notice:
```
server # qemu-kvm: at most 2047 MB RAM can be simulated
```
2. Hard fail
```
self.shell.send(out_command.encode())
BrokenPipeError: [Errno 32] Broken pipe
```
(Took me a while to consider those lines are related)
the non-networkd backend does not wait for slaac to finish (ie, ipv6
addresses coming out of tentative state), and that breaks the mosquitto
bind_interface test slightly. if slaac takes too long the test will run
into mosquitto restart limits and fail.
