Previously, the osbuild executor had its internal temporary directory that
served as the output directory. However, this approach gives no power to
the caller to control the lifetime of the produced artifacts. When more
images are built using one executor, the results will accumulate in one
place possibly leading to exhaustion of disk space.
This commit removes the executor's internal output directory. The output
directory can now be passed to osbuild.compile, so the caller can control
its lifetime. If no directory is passed in, the compile method will use
its own temporary directory - this is useful in cases when the caller
doesn't care about the built artifacts or the manifest doesn't have any
outputs.
