autoinstall is not executing postruncmd commands in user-data section

attaching server debug log and here's the current entirety of user-data:

```
  user-data:
    write_files:
      - content: |
          PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
          LC_ALL="en_US.UTF-8"
          LANG="en_US.UTF-8"
          LANGUAGE="en_US:en"
          EDITOR="vim"
        owner: "root:root"
        path: /etc/environment
        permissions: "0644"
      - content: |
          config:
            core.https_address: '[::]:8443'
            core.trust_password: P@ssw0rd!
          networks: []
          storage_pools:
          - config:
              source: local
              volatile.initial_source: /dev/vdb
              zfs.pool_name: local
            description: "Local ZFS pool on /dev/sdb"
            name: local
            driver: zfs
          profiles:
          - config: {}
            description: Default LXD profile
            devices:
              eth0:
                name: eth0
                nictype: bridged
                parent: br0
                type: nic
              root:
                path: /
                pool: local
                type: disk
            name: default
          projects:
          - config:
              features.images: "true"
              features.networks: "true"
              features.profiles: "true"
              features.storage.volumes: "true"
            description: Default LXD project
            name: default
        owner: "root:root"
        path: /root/lxd-config
        permissions: "0640"
    packages:
      - net-tools
      - bridge-utils
# - zfsutils-linux
    postruncmd:
      - echo hello > /root/test1
      - zpool destroy local
      - sleep 10
      - apt install zfsutils-linux -y
      - zpool create -f -m none -O compression=on local /dev/vdb
      - cat /root/lxd-config | lxd init --preseed

```

Found this bug while looking at postruncmd today.

It seems that preruncmd/postruncmd is a juju special config key, it's not clear from the commit history, but I believe it exists because juju itself uses bootcmd/runcmd so wants you to use those special keys which it then executes.

So these keys won't work in subiquity or any non-juju environment. 'postruncmd' doesn't exist in the cloud-init repository, only juju's