install: support passed `-v /sysroot:/target` for ostree/bootc systems

Inspired by https://github.com/bootc-dev/bootc/pull/1752#discussion_r2665100260

Copy comments here:
```
And for ostree/bootc systems, that's always the mounted deployment root conventionally today when they say -v /:/target.

But that's just conventional - what would happen if they passed -v /sysroot:/target say?

The more I think about this the more I feel what we should do is detect when the passed root filesystem refers to either
a mounted deployment root or the physical root, and automatically canonicalize internally to "physical root" and
"deployment root" (which for non-ostree systems are the same thing), and then discard the user provided input path.
```

Need to do some investigation for this.