Add `x_version=src`.

[supakeen]

So this is a bit of a bigger issue, currently various arguments are given to the Makefile, for example: make run osbuild_version=v55 osbuild_composer_version=v54. These are translated to git checkouts when creating the build containers and then passed along all the parts as identifiers for which containers to run.

When x_version=src is given then a second argument of x_src=/path should be given. This should create a container which mounts in the x_src directory for that given target so we can build everything directly from a local folder.