You have 2 ways of doing this:
flex-grow: 0 (fiddle)
If you set flex-grow to 0 you are telling the items to keep their width and expand the empty space. Using flex-grow: 1 the items will expand the width to fit the space.
invisible items (fiddle)
Add some invisible items after the normal items with the same flex properties. You will get a left aligned look.
In this solution there will be the same number of items in all visible rows so make sure to add enough invisible items to work properly on larger screens.