integcli: speed up TestBuildSixtySteps

This improves the TestBuildSixtySteps test by switching from busybox to
scratch and simply adding a file.

This lowers the execution time of that test from 20 seconds to 5
seconds.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5871 from unclejack/speed_up_buildsixtysteps

integcli: speed up TestBuildSixtySteps