build_folder is serialized

[philip-paul-mueller]

SDFG.build_folder was not serialized, which was not a problem if it was not set explicitly.
If set explicitly it was also not serialized, thus the following code will fail:

sdfg = make_an_awesome_sdfg()
sdfg.build_folder = "/my/special/build_folder"
dolly = dace.SDFG.from_json(sdfg.to_json())
assert dolly.build_folder == sdfg.build_folder

This PR serialized build_folder is set otherwise it is not included, ensures compatibility with older versions.