fix(ci): put a default back into $tfa_downloads

Commit ebfc4a05 removed the default and instead expects it to be set in
a CI variable. Well this is not valid locally, where we still want the
default.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Iafe43cd51ad62f9445feeaa0723c713638ea9d57
1 file changed