Improve gradle spotless apply suggestion

[breskeby]

This PR adds a minor improvement to the suggestion message

Run 'gradlew spotlessApply' to fix these violations.

to

1. use a OS specific invocation of gradlew
2. provide the full task path of the spotlessApply task

which results in

// windows
Run 'gradlew.bat :spotlessApply' to fix these violations.

// unix, osx, linux
Run './gradlew :spotlessApply' to fix these violations.

Especially for larger multi project builds, this makes copy pasting the suggested fix easier and more efficient as it only runs on the problematic project and not on all projects in the multi project build.

[nedtwigg]

Thanks! A few small problems noted below. Main problem is that our CI didn't run on this PR, which I believe I just fixed.

I assume that you are working on a very large project. Does it happen often that you have problems in two+ projects? The ultimate form of this PR would be to somehow aggregate failures across all invocations of spotlessCheck, and combine it into one error message. I'm definitely not asking for that PR, and infact we would reject it at the moment because we need to merge #576 first, but I am curious if you feel you would benefit from it or not.

[breskeby]

I was thinking if gathering all tasks end have one message that tells you all the paths to a apply would be a good solution here. In general that would only apply if you ran gradle with --continue or maybe with running --parallel. So far me personally never hit the issue of having many different spots to fix at once but there is a use case for that. With this solution here, you now would get one suggested fix per failing task (project) with the appropriate fix. Given the simple implementation that is a good compromise I think.
One option could be to move the messaging to the buildFinished hook and check all the failing tasks and then provide an even better suited message (e.g. based on how many spotless tasks have failed or how big the project is). But I'm not sure if this is worth the added complexity tbh

[breskeby]

I've rebased and updated the PR against latest master

[nedtwigg]

Gonna let this breathe for a couple days before merging and releasing.

[nedtwigg]

The OS fix makes it easier to merge :)

[nedtwigg]

Released in plugin-gradle 4.0.0

[nedtwigg]

I don't know how to detect this, but Windows PowerShell requires .\gradlew or .\gradlew.bat, the suggestion here won't work. Doesn't bother me, just an FYI for any interested parties :)