Only set environment variables if not already set

Currently, the run.sh script will take the environment variables from
rancher-system-agent.env, overwriting any variables that are already
set. This makes changing any environment variables that are in the env
file impossible.

After this change, only the variables that are not set in the
environment are set from the rancher-system-agent.env file.

package/suc/run.sh

@@ -36,7 +36,14 @@ fi
 export CATTLE_AGENT_BINARY_LOCAL=true
 export CATTLE_AGENT_BINARY_LOCAL_LOCATION=${TMPDIR}/rancher-system-agent
 if [ -s /host/etc/systemd/system/rancher-system-agent.env ]; then
-  export $(grep -v '^#' /host/etc/systemd/system/rancher-system-agent.env | xargs)
+  for line in $(grep -v '^#' /host/etc/systemd/system/rancher-system-agent.env); do
+    var=${line%%=*}
+    val=${line##*=}
+    eval v=\"\$$var\"
+    if [ -z "$v" ]; then
+      export "$var=$val"
+    fi
+  done
 fi
 chroot /host ${TMPDIR}/install.sh "$@"