Support environment variables in Maven settings.xml

## Description
maven settings file supports envs - https://maven.apache.org/settings.html#Properties
e.g. 
```
      <privateKey>${user.home}/.ssh/id_dsa</privateKey>
```

So we need to add support for decoding `${env.*}`


### Discussed in https://github.com/aquasecurity/trivy/discussions/7070
