Install config file are not sourced from the container with `--src-imgref`

So I know this is a bit tricky but i am still filing this because maybe there is a solution I don't know about.

When calling `bootc install .... --src-imgref`, i.e. running bootc outside the container we want to install, install config files that are shipped into the container aren't sourced.

Note image builder works around this by extracting the files from the container into the buildroot before invoking bootc install.

see also https://github.com/bootc-dev/bootc/issues/1410