Issue #120 fix - set fork working dir to temp dir

[hesselapplications]

Fixes https://github.com/springdoc/springdoc-openapi-gradle-plugin/issues/120

By default gradle-execfork-plugin sets the working dir to the project directory. This causes issues when running generateOpenApiDocs during a gradle build:

Reason: Task ':skeleton-server:forkedSpringBootRun' uses this output of task ':skeleton-server:bootJarMainClassName' without declaring an explicit or implicit dependency. This can lead to incorrect results being produced, depending on what order the tasks are executed.

Using a temporary directory instead of the project dir default resolves this