investigate: why does DetSys nix installer fail on namespace builders

[colemickens]

ref: 61cb3f5

and: https://github.com/nix-community/flake-firefox-nightly/actions/runs/6251954682/job/16974155319

[colemickens]

GitHub doesn't really seem to prune action runs but just in case:

Fetching binary from https://install.determinate.systems/nix/nix-installer-x86_64-linux?ci=github
Downloaded `nix-installer` to `/tmp/nix-installer-u1xeoe/nix-installer-x86_64-linux`
Execution environment: {
    "NIX_INSTALLER_NO_CONFIRM": "true",
    "NIX_INSTALLER_MODIFY_PROFILE": "true",
    "NIX_INSTALLER_DIAGNOSTIC_ENDPOINT": "https://install.determinate.systems/nix/diagnostic",
    "NIX_INSTALLER_START_DAEMON": "true",
    "NIX_INSTALLER_EXTRA_CONF": "access-tokens = github.com=***\ntrusted-users = root runner\naccess-tokens = github.com=***\n"
}
`nix-installer` needs to run as `root`, attempting to escalate now via `sudo`...
systemd was not active.

If it will be started later consider, passing `--no-start-daemon`.

To use a `root`-only Nix install, consider passing `--init none`.
Error: Error: Non-zero exit code of `1` detected

[colemickens]

unrelated, but just somewhere to stick it for now:

• does cachix/install-nix-action sniff out KVM support and enable it? should it?
• same question for DeterminateSystems/nix-installer-action