Refactor code to make linter runs succeed

This commit refactors the codebase in order to make the shellcheck
workflow pass.

duplicati/duplicati-backup.sh

@@ -22,7 +22,7 @@ while getopts ":f:h:u:p:m:b:" opt; do
     f) SOURCE_DIR="$OPTARG";;
     h) SSH_DESTINATION="$OPTARG";;
     u) SSH_USERNAME="$OPTARG";;
-    p) SSH_KEY="$OPTARG";;
+    p) SSH_PASSWORD="$OPTARG";;
     m) MAX_BACKUPS="$OPTARG";;
     b) BACKUP_FOLDER="$OPTARG";;
     \?) echo "Invalid option: -$OPTARG"; exit 1;;