importer: chdir back to the previous cwd when building the sources
importer: chdir back to the previous cwd when building the sources

If we don't do that, at some point later in the process `files/git/last`
is removed, resulting in an invalid cwd, and sometime later some
submethod calls it, resulting in uncatched exception, crash, horror and
despair.

jadonk/debexpo,jadonk/debexpo,jadonk/debexpo