Use environment variable for download server

To ensure tf-a-jobs 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

Signed-off-by: Saheer Babu <saheer.babu@arm.com>

Change-Id: Ibbb7bf98c087e12617b14481cfa0e5a848cf8c8e
diff --git a/tf-a-coverity.yaml b/tf-a-coverity.yaml
index 57a30d4..bcdab21 100644
--- a/tf-a-coverity.yaml
+++ b/tf-a-coverity.yaml
@@ -62,7 +62,7 @@
 
             # Fetch coverity tool and untar it
             cd ${WORKSPACE}
-            wget https://downloads.trustedfirmware.org/tf-a/tf-a-coverity/coverity_tool.tgz
+            wget ${DOWNLOAD_SERVER_URL}/tf-a/tf-a-coverity/coverity_tool.tgz
             tar -xzf coverity_tool.tgz
             mv cov-analysis-linux64-${COVERITY_VERSION} coverity
             export PATH=${WORKSPACE}/coverity/bin:${PATH}