Read Gradle build dir in config cache compatible way

[DSteve595]

This makes the task compatible with Gradle Configuration Cache, and enables that in the sample. Needed for enabling config cache in godot-kotlin-jvm.

[Copilot]

Pull Request Overview

This PR makes the Gradle plugin compatible with Gradle Configuration Cache by replacing direct access to project.buildDir with the configuration cache-safe project.layout.buildDirectory provider API.

• Replaces deprecated project.buildDir with configuration cache-compatible provider API
• Enables Gradle Configuration Cache in the sample project configuration

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
samples/gradle.properties	Enables Gradle Configuration Cache in the sample project
kotlin-preprocessors/src/main/kotlin/com/utopiarise/kotlin/preprocessors/gradle/PreProcessorPlugin.kt	Updates task to use configuration cache-compatible build directory provider

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