Add flag to preserve build tree for interactive inspection

[msehnout]

In case the stage or assembler fails, the build tree is cleaned up
automatically, this option allow us to examine the tree in interactive
bash before it gets removed.

[larskarlitski]

Interesting! I see two problems:

1. The keepalive option doesn't work when check=False.
2. It won't work at all if bash isn't (yet) installed

I wonder if we should just save the tree on failure and print its id?

[msehnout]

Interesting! I see two problems:

1. The `keepalive` option doesn't work when `check=False`.

Yes, but I can't see any usage of check=False in the code? Maybe we can force check=True when keepalive=True?

2. It won't work at all if `bash` isn't (yet) installed

I wonder if we should just save the tree on failure and print its id?

Yes, that would be nice. Otherwise I would just let osbuild fail in case bash is not yet installed. We can fix it once there is a problem with it.

[teg]

This goes a bit in the direction of the debug mode we had before (trying to allow you to enter the namespace using machinectl). I think something like this is useful, but I don't think we should spawn bash. Instead, we should simply wait for the user to Ctrl-C the failed stage, and give them the information needed to access the tree from the outside (its path). It may also make sense to pin the relevant namespaces as files in some discoverable place so that we could make a tool to introspect them. Details are still fuzzy to me, but take home is: don't rely on bash being in the target, and don't spawn bash at all, just give the path of the tree for the caller (or some secondary tool we make) to do with what they wil.

[msehnout]

Sure, we don't need to use this approach. It is just something I used when I was debugging my stages and I wanted to share the idea, but I'm all in for something more generally useful.

[teg]

Makes sense! I'll try to file another PR along the lines of what I outlined above.