[BUG] Gradle plugin: incremental build does not work if `outputDir` is not in `$buildDir`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When I use the GenerateTask and want to target an outputDir which path is not in the $buildDir the client I'm generating is generated on each build and I don't benefit from incremental builds (UP-TO-DATE flag on task).
I'm wondering if this is something related to Gradle our this plugin specifically and if there's an existing workaround.

If I use the $buildDir in the outputDir path the incremental build works, and my client get generated only upon spec updates.

openapi-generator version

5.3.1

Steps to reproduce

No incremental builds with the following build.gradle

import org.openapitools.generator.gradle.plugin.tasks.GenerateTask

plugins {
    id "org.openapi.generator" version "5.3.1"
    id 'airbyte-python'
    id 'airbyte-docker'
}

airbytePython {
    moduleDirectory 'octavia_cli'
}


task generateApiClient(type: GenerateTask) {
    inputSpec =  "$rootDir.absolutePath/airbyte-api/src/main/openapi/config.yaml"
    outputDir = "$projectDir/airbyte_api_client"

    generatorName = "python"
    packageName = "airbyte_api_client"
}

Works as expected when using $buildDir in the outputDir path:

import org.openapitools.generator.gradle.plugin.tasks.GenerateTask

plugins {
    id "org.openapi.generator" version "5.3.1"
    id 'airbyte-python'
    id 'airbyte-docker'
}

airbytePython {
    moduleDirectory 'octavia_cli'
}

task generateApiClient(type: GenerateTask) {
inputSpec = "$rootDir.absolutePath/airbyte-api/src/main/openapi/config.yaml"
outputDir = "$buildDir/airbyte_api_client"

generatorName = "python"
packageName = "airbyte_api_client"

}

Related issues/PRs

@HenningWaack PR's https://github.com/OpenAPITools/openapi-generator/pull/6716 introduced support for incremental build.

Start with the Gradle plugin's GenerateTask and the incremental-build support introduced by PR 6716. Reproduce the build.gradle examples with outputDir outside and inside $buildDir, then trace how outputs are tracked. Done means the task is UP-TO-DATE outside $buildDir when the input specification is unchanged.