Use the directory mkosi was invoked in as the default for BuildSources=

Use the directory mkosi was invoked in as the default for BuildSources=

While parsing config, we use chdir(). Also, when a BuildSources=
match is found, BuildSources= is initialized to its default value
which is Path.cwd(). However, we want the default value to be the
top level directory that mkosi was invoked in, not the current working
directory that we happen to be in while parsing configuration. Let's
fix this by using the directory mkosi was invoked in instead of Path.cwd().