Add a persistent "Add build step" button that is available to insert an item between existing items. This button should be intuitively placed within the interface, making it easy for users to insert items at specific positions without the need to move the steps later.

 
This could be a more generic feature, not limited to build steps. For instance, allowing the addition of triggers between other build triggers.

Attached is an example of how I envision the button placement. The plus icon could expand to reveal a full button upon hover, or a title-tooltip could be utilized.