piuparts_wrapper: enable --mirror parameter

We can't rely on what's defined in the base tgz file, since the environment might still refer to
a mirror that no longer exists as such. For example with current wheezy repository (that has been
moved to archive.debian.org) piuparts runs might fail with:

| W: Failed to fetch http://deb.debian.org/debian/dists/wheezy/main/binary-i386/Packages 404 Not Found
| [...]
| E: Some index files failed to download. They have been ignored, or old ones used instead.

scripts/piuparts_wrapper

@@ -122,5 +122,6 @@ piuparts \
   --log-file='piuparts.txt' \
   ${scriptsdir:+--scriptsdir "$scriptsdir"} \
   ${piuparts_tmpdir:+--tmpdir="$piuparts_tmpdir"} \
+  ${MIRROR:+--mirror="$MIRROR"} \
   ${basetgz:-} \
   "$@"