Add Acquire::AllowInsecureRepositories to fix apt-secure in sid

The apt-secure option does not work anymore when building a sid image, as with apt 1.6 the existing options are no longer enough to get apt to accept an unsigned repository, which is necessary when using a local cached repository (offline build).

Pass Acquire::AllowInsecureRepositories "true"; together with the other options when --apt-secure false is used to fix the issue.