sed command in auto install fails in certain circumstance

[Matti Kaupenjohann]

If I want to insert or append a line in the `/etc/aliases` file in the section `late-commands` from ubuntu auto-install, like the follwoing:

```
late-commands:
  - sed -i '/^postmaster:.*/i root: root' /target/etc/aliases
```

I will get the following error:

```
finish: subiquity/Late/load_autoinstall_data: {"sed -i -e '/^postmaster:.*/i\\'$'root": "root' /target/etc/aliases"} is not of type 'string', 'array'
```

From the the double quotes it seems, instead of running the complete command, the part in front of the colon will be interpreted as key. In this scenario the key:value regex seems to completly negate the single quotes of the `sed` command.

[Michael Hudson-Doyle (mwhudson)]

This is just YAML being bad I think? Either of the following should work:

late-commands:
  - |
    sed -i '/^postmaster:.*/i root: root' /target/etc/aliases
  - - sed
    - -i
    - "/^postmaster:.*/i root: root"
    - /target/etc/aliases

[Launchpad Janitor (janitor)]

[Expired for subiquity (Ubuntu) because there has been no activity for 60 days.]