Fixes Configuration caching issues

[stephanenicolas]

Using versions: [Kotlin:1.4-M3, Gradle:6.5, AGP 4.1 beta 04], I could detect some issues when running instant execution:

428 problems were found reusing the configuration cache, 4 of which seem unique.
- field '$disableIncrementalCompilationTaskProvider' from type 'com.squareup.hephaestus.plugin.HephaestusPlugin$disableIncrementalKotlinCompilation$disableIncrementalCompilationAction$1$1': cannot deserialize object of type 'org.gradle.api.Task' as these are not supported with the configuration cache.
  See https://docs.gradle.org/6.6-rc-1/userguide/configuration_cache.html#config_cache:requirements:task_access
- field '$disableIncrementalCompilationTaskProvider' from type 'com.squareup.hephaestus.plugin.HephaestusPlugin$disableIncrementalKotlinCompilation$disableIncrementalCompilationAction$1$1': value 'undefined' is not assignable to 'org.gradle.api.tasks.TaskProvider'
- field '$project' from type 'com.squareup.hephaestus.plugin.HephaestusPlugin$disableIncrementalKotlinCompilation$disableIncrementalCompilationAction$1': cannot deserialize object of type 'org.gradle.api.Project' as these are not supported with the configuration cache.
  See https://docs.gradle.org/6.6-rc-1/userguide/configuration_cache.html#config_cache:requirements:disallowed_types
- field '$project' from type 'com.squareup.hephaestus.plugin.HephaestusPlugin$disablePreciseJavaTracking$1': cannot deserialize object of type 'org.gradle.api.Project' as these are not supported with the configuration cache.
  See https://docs.gradle.org/6.6-rc-1/userguide/configuration_cache.html#config_cache:requirements:disallowed_types

This PR fixes the various issues. Thx to @bamboo for the guidance

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[stephanenicolas]

It passed and I don't see the error anymore.

[vRallev]

An abstract class is automatically open. open is redundant.

[vRallev]

You call this too early. It has to happen in the doFirst, because otherwise the task that toggles this flag didn't run, yet.

[vRallev]

Move this empty line above the comment.

[vRallev]

Tested and confirmed locally that incremental builds still work as expected.