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: Ia8f141fe03dc1f68990bcc4aeea2e90019f9875b
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
diff --git a/script/tf-coverity/run_coverity_on_tf.py b/script/tf-coverity/run_coverity_on_tf.py
index 457f38b..c4cca31 100755
--- a/script/tf-coverity/run_coverity_on_tf.py
+++ b/script/tf-coverity/run_coverity_on_tf.py
@@ -37,7 +37,7 @@
 
 def get_coverity_tool():
     coverity_tarball = "coverity_tool.tgz"
-    url = "https://downloads.trustedfirmware.org/tf-a/tf-a-coverity/" + coverity_tarball
+    url = "${DOWNLOAD_SERVER_URL}/tf-a/tf-a-coverity/" + coverity_tarball
     print("Downloading Coverity Build tool from %s..." % url)
     file_handle = urllib.request.urlopen(url)
     output = open(coverity_tarball, "wb")