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
diff --git a/arm-env.sh b/arm-env.sh
index 0930990..fc59b65 100644
--- a/arm-env.sh
+++ b/arm-env.sh
@@ -11,7 +11,8 @@
 
 nfs_volume="/arm"
 jenkins_url="https://jenkins.oss.arm.com"
-tfa_downloads="${DOWNLOAD_SERVER_URL}/tf-a"
+DOWNLOAD_SERVER_URL="${DOWNLOAD_SERVER_URL:+${DOWNLOAD_SERVER_URL}/tf-a}"
+tfa_downloads="${DOWNLOAD_SERVER_URL:-https://downloads.trustedfirmware.org/tf-a}"
 ci_env="armci"
 
 # Source repositories.