gps2: fix logging call

The multiprocessing workers return a list already, no need to embed that
in another one.

This should change logging from
generate-packages-sources2|dak|['generate-packages', 'unstable', 'main', 's390x']
to
generate-packages-sources2|dak|generate-packages|unstable|main|s390x

Signed-off-by: Julien Cristau <jcristau@debian.org>