Add a recap of what SwiftLint did in run_swiftlint

[mokagio]

This is a bit of a nitpick, if we're honest. But... I just reviewed wordpress-mobile/WordPressAuthenticator-iOS#839 and looked at the logs of the new SwiftLint CI step

From there, it's not clear what the step does other than downloading the image from Docker. This is because we are in the happy path and no warning or error was raised. Still, it would be good to see a log like

SwiftLint finished with exit status 0.