Properly update /etc/apk/repositories when the mirrors change

This is important for the binary repository scripts, so it's feasible
to test the binary package build and challenge process locally without
setting up a new chroot whenever changing the repo URLs.

Also it behaves a bit more intuitively, because it really uses the
repo URL specified on the commandline, even when the chroot is already
set up.

pmb/parse/arguments.py

@@ -225,6 +225,7 @@ def arguments():
                             "apkindex_files": {},
                             "apkbuild": {},
                             "apk_min_version_checked": [],
+                            "apk_repository_list_updated": [],
                             "aports_files_out_of_sync_with_git": None,
                             "find_aport": {}})