Don't adjust WORKSPACE env variable if already set by caller

The script incorrectly adjusts the WORKSPACE env variable.  This change
fixes that.

Change-Id: I05cbcafb0d27090171fb01d3b2a7c1c15b9a5b1a
Signed-off-by: Andrew Geissler <andrewg@us.ibm.com>
diff --git a/run-qemu-robot-test.sh b/run-qemu-robot-test.sh
index 1682500..9becd9c 100755
--- a/run-qemu-robot-test.sh
+++ b/run-qemu-robot-test.sh
@@ -10,7 +10,7 @@
 set -uo pipefail
 
 QEMU_RUN_TIMER=300
-WORKSPACE=${WORKSPACE:-${HOME}}/${RANDOM}${RANDOM}
+WORKSPACE=${WORKSPACE:-${HOME}/${RANDOM}${RANDOM}}
 DOCKER_IMG_NAME="openbmc/ubuntu-robot-qemu"
 
 # Get base directory of our repo so we can find the scripts later