Config service should check whether the program is running in the app's snap
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Issue body
### Bug Description
https://github.com/canonical/craft-application/blob/86fcd4078907cf084367d38fceda1b8a52b97911/craft_application/services/config.py#L91-L97
Currently we just check if we're running in *a* snap, we should check if we're running in the app's snap.
### To Reproduce
Set the `SNAP` environment variable
### part yaml
```shell
```
### Relevant log output
```shell
n/a
```
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