Cannot access TestLink environment variables within Jenkins build

I'm configuring my first Jenkins / TestLink job. I don't seem to have access to any of the TestLink environment variables. I have pared down my Jenkins job to the following two statements under Build / Invoke TestLink / Test Execution / Single Build Steps / Execute Shell:

echo "Test plan name is: $TESTLINK_TESTPLAN_NAME"
echo "Build number is: $BUILD_NUMBER"

The Jenkins console output is as follows:

Started by user lisas
Building in workspace /var/lib/jenkins/jobs/Website-Linkchecker-TestLink-integration/workspace
Preparing TestLink client API.
Using TestLink URL: http://localhost/testlink/lib/api/xmlrpc.php

Found 1 automated test cases in TestLink.

Sorting automated test cases by TestLink test plan execution order.

Executing single Build Steps.

[workspace] $ /bin/sh -xe /tmp/hudson8927570372825500742.sh
+ echo Test plan name is:
Test plan name is:
+ echo Build number is: 8
Build number is: 8
Executing iterative Build Steps.

Looking for the test results of TestLink test cases.

Found 0 test result(s).

Finished: SUCCESS

As you can see, the $BUILD_NUMBER environment variable is translated as expected. The $TESTLINK_TESTPLAN_NAME environment variable is not.

Any hints as to what might have gone wrong?