RFR: 8263204: Add Gradle Wrapper Validation Action

[John Neffenger]

On Mon, 8 Mar 2021 22:37:10 GMT, John Neffenger <github.com+1413266+jgneff at openjdk.org> wrote:

>> If it isn't too hard to make the running of the other three jobs dependent on the wrapper validation job, that would be good, as long as the validation job is fast. The other three still need to run in parallel with each other.
>> 
>> Alternatively, you could make the wrapper validation a task that is replicated in each of the three jobs.
>
> Okay, this is looking better! The validation took only 23 seconds, and GitHub shows the following results:
> 
> ![some-checks-were-not-successful-2](https://user-images.githubusercontent.com/1413266/110390566-0b7ac680-801b-11eb-8090-f1548046094e.png)
> 
> The screenshot below shows the notification I received by e-mail:
> 
> ![run-failed-message](https://user-images.githubusercontent.com/1413266/110390602-17ff1f00-801b-11eb-9870-c5f955b4739c.png)
> 
> Now we'll see whether the three build jobs still run in parallel when the wrapper validation succeeds. My next commit will restore the Gradle Wrapper JAR file.

Looks good: two preliminary checks followed conditionally by the three concurrent build jobs:

![some-checks-haven’t-completed-yet](https://user-images.githubusercontent.com/1413266/110392468-d15ef400-801d-11eb-8a1c-1bf49ed32a2a.png)

Once the build jobs finish, this pull request is finally ready for review. Thanks for your timely help, Kevin.

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PR: https://git.openjdk.java.net/jfx/pull/419