snapcraft doesn't allow setting the environment for hooks

[Ian Johnson]

With the following hook environment specification in snapcraft.yaml:

```
hooks:
  install:
    environment:
      LC_ALL: C.UTF-8
      LANG: C.UTF-8
```

snapcraft fails like this:

```
$ snapcraft
Issues while validating snapcraft.yaml: The 'hooks/install' property does not match the required schema: Additional properties are not allowed ('environment' was unexpected)
```

This is with version 3.8 on amd64:

```
$ snapcraft version
snapcraft, version 3.8
$ snap info snapcraft | grep installed
installed: 3.8 (3440) 60MB classic
```