Cannot apply model-defaults in isomorphic manner

[Vitaly Antonenko]

One of our tests checks applying model-defaults in an isomorphic manner:

```
juju model-defaults "${BOOTSTRAPPED_CLOUD}" --format=yaml | juju model-defaults "${BOOTSTRAPPED_CLOUD}" --ignore-read-only-fields --file -
```

It looks like this commit breaks working with pipe inputs: https://github.com/juju/juju/commit/caa28090

We get the following error on output:
```
anvial@ubuntu-pc:~/Projects/juju-dev$ juju model-defaults lxd --format=yaml | juju model-defaults lxd --ignore-read-only-fields --file -
ERROR cannot use --file flag and get value simultaneously

```

We need to fix it to make our 3.1 CI green.

[Harry Pidcock (hpidcock)]

https://github.com/juju/juju/pull/15718