Port to rustix, drop nix

[cgwalters]

We already have rustix as a dependency; now that we don't use IPC anymore it's trivial to port to rustix. I prefer its API and maintainer.

[travier]

The syncfs rework is tricky and I'm not sure I fully understand it but LGTM overall.

[travier]

CI failing on:

sudo skopeo copy docker-daemon:localhost/bootupd:latest containers-storage:localhost/bootupd:latest
  shell: /usr/bin/bash -e {0}
  env:
    CARGO_TERM_COLOR: always
    CACHE_ON_FAILURE: false
    CARGO_INCREMENTAL: 0
time="202[4](https://github.com/coreos/bootupd/actions/runs/9597160268/job/26465639664?pr=669#step:6:4)-06-20T12:14:17Z" level=info msg="Not using native diff for overlay, this may cause degraded performance for building images: kernel has CONFIG_OVERLAY_FS_REDIRECT_DIR enabled"
time="2024-0[6](https://github.com/coreos/bootupd/actions/runs/9597160268/job/26465639664?pr=669#step:6:6)-20T12:14:17Z" level=info msg="[graphdriver] using prior storage driver: overlay"
time="2024-06-20T12:14:1[7](https://github.com/coreos/bootupd/actions/runs/9597160268/job/26465639664?pr=669#step:6:7)Z" level=fatal msg="initializing source docker-daemon:localhost/bootupd:latest: loading image from docker engine: Error response from daemon: {\"message\":\"client version 1.22 is too old. Minimum supported API version is 1.24, please upgrade your client to a newer version\"}"

similarly in #669.

[travier]

Looks like we need a new skopeo in the GitHub runner images :/

[travier]

containers/skopeo#2353