scripts: when downloading, use aria2 only when //

Even when // downloads are not enabled, aria2 can
fail on some servers (eg. uclibc.org).

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Compute IA-64-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

}