Use environment variable for download server

To ensure tf-a-ci-scripts can run on both OpenCI
and Arm CI, replace ci.trustedfirmware.org with
enironment variable.

DOWNLOAD_SERVER_URL is a system-wide environment
variable and was configured in:
Manage Jenkins > System > Global Properties

Change-Id: I53bbb8766c9264c7cc1200f8884324b6650f31da
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
diff --git a/openci-env.sh b/openci-env.sh
index 9b75ab3..1e6a554 100644
--- a/openci-env.sh
+++ b/openci-env.sh
@@ -11,11 +11,11 @@
 
 nfs_volume="${WORKSPACE:?}/nfs"
 jenkins_url="${JENKINS_PUBLIC_URL}"
-tfa_downloads="https://downloads.trustedfirmware.org/tf-a"
+tfa_downloads="${DOWNLOAD_SERVER_URL}/tf-a"
 ci_env="openci"
 
 tfa_branch=${TF_GERRIT_BRANCH##*/}
 if echo $tfa_branch | grep -q "^lts-v"; then
     # LTS branch, change the download space to the respective one
-    tfa_downloads="https://downloads.trustedfirmware.org/tf-a-$tfa_branch"
+    tfa_downloads="${DOWNLOAD_SERVER_URL}/tf-a-$tfa_branch"
 fi