build_image still too aggressive with right sizing (part troix)

Regression introduced by c3d408e3

Found a case where 1 + 1/60 reduction in size based on free space
failed when right sizing an ext4 image. Reduced the math to 1 + 1/61.

Test: compile
Bug: 111302946
Change-Id: I0f32c75e80b5da4d47671055ac274ccc2b485992