Fix ShellCheck

Author: hugovk

.ci/install.sh

@@ -2,12 +2,12 @@
 
 aptget_update()
 {
-    if [ ! -z $1 ]; then
+    if [ -n "$1" ]; then
         echo ""
         echo "Retrying apt-get update..."
         echo ""
     fi
-    output=`sudo apt-get update 2>&1`
+    output=$(sudo apt-get update 2>&1)
     echo "$output"
     if [[ $output == *[WE]:\ * ]]; then
         return 1