Fix env pass for ubuntu-16.04

Change-Id: I317f5f46ca8beeb0d44bf66364af8b2a51d67d2f
diff --git a/common/ubuntu_base.sh b/common/ubuntu_base.sh
index aafae3a..bca322d 100644
--- a/common/ubuntu_base.sh
+++ b/common/ubuntu_base.sh
@@ -1,6 +1,6 @@
 #!/bin/bash -xe
 
-UBUNTU_BASEURL="${UBUNTU_BASEURL:-"mirror://mirrors.ubuntu.com/mirrors.txt"}"
+UBUNTU_BASEURL="${UBUNTU_BASEURL:-mirror://mirrors.ubuntu.com/mirrors.txt}"
 
 ## Base packages and setup
 export DEBIAN_FRONTEND=noninteractive