fix dash compatibility

Author: kvaps

kubectl-node_shell

@@ -56,14 +56,14 @@ while [ $# -gt 0 ]; do
 done
 
 while [ $# -gt 0 ]; do
-  cmd+="\"$1\", "
+  cmd="$cmd\"$1\", "
   custom=true
   shift
 done
 if [ "$custom" = true ]; then
   cmd="$(echo "$cmd" | awk '1' RS=', \n') ]"
 else
-  cmd+='"bash", "-l" ]'
+  cmd="$cmd\"bash\", \"-l\" ]"
 fi
 
 if [ -z "$node" ]; then