Fix the Gradle plugin not to break the clean task when Gradle's configuration cache is enabled

[eskatos]

Using the project at execution time is not allowed when Gradle's configuration cache is enabled.
https://docs.gradle.org/current/userguide/configuration_cache.html#config_cache:requirements:use_project_during_execution

Before this commit, the spotless plugin registered a doLast task action on the clean task that accessed the projet at execution time. The effect of this is to break using the clean task on all builds that have the Spotless plugin applied when the configuration cache is enabled.

This manifests the following way:

user@host $ ./gradlew --configuration-cache clean
Configuration cache is an incubating feature.

FAILURE: Build failed with an exception.

* What went wrong:
Configuration cache problems found in this build.

1 problem was found storing the configuration cache.
- Task `:clean` of type `org.gradle.api.tasks.Delete`: cannot serialize object of type 'org.gradle.api.internal.project.DefaultProject', a subtype of 'org.gradle.api.Project', as these are not supported with the configuration cache.
  See https://docs.gradle.org/7.0-20210211110842+0000/userguide/configuration_cache.html#config_cache:requirements:disallowed_types

See the complete report at file:///..../configuration-cache-report.html

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 1s
1 actionable task: 1 executed

Here is a ZIP of the HTML configuration cache report for your curiosity: configuration-cache-report.zip

This PR changes the way the Spotless plugin configures the clean task to not access a project in the added doLast task action by computing the Spotless cache key upfront at configuration time. The effect is that builds with the Spotless plugin applied can now run the clean task with the configuration cache enabled.

This is a fix for #796

[nedtwigg]

Thanks very much!

[nedtwigg]

Published in 5.10.1