Environment variable not expanded in packages entries

[jeremie]

Environment variables aren't expanded when used in packages entries, e.g. the following fails,

```
pipeline:
- test

jobs:
    test:
        series: focal
        architectures: amd64
        run: echo hello world >output
        environment: {"mypackage": "file"}
        packages: [$mypackage]
```

Note that adding quotes/ticks doesn't help.

[Jürgen Gmach (jugmac00)]

We had no intention to do that, at least not in the beginning, so I regard this a feature request.

Similar to YAML aliases and YAML includes, I would like to direct the discussion to https://chat.canonical.com/canonical/pl/r71cgk3ejtr1zmbfnfitbi187c where I suggested to use an external client side tool / script / one-liner to do all expansion, so there would be no changes necessary to the CI runner.