How can I specify local gradle.properties per project?

One possible solution:

1. Define an alias for custom JDK, e.g.: jdk-8, jdk-11
2. All contributors should specify paths to those JDK via build environment (any way from Gradle Docs). For example:

// file ~/.gradle/gradle.properties
jdk-8=/path/to/jdk8
jdk-11=/path/to/jdk11

jdk-8 and jdk-11 will be resolved as properties by Gradle in any build.

3. Configure javaCompile task for specific projects in root build.gradle.kts:

fun Project.setupCustomJdk(jdkAlias: String) {
    findProperty(jdkAlias)?.toString()?.trim()?.let { customJdkPath ->
        tasks.withType<JavaCompile> {
            options.isFork = true
            options.forkOptions.javaHome = File(customJdkPath)
            logger.info("Task javaCompile using JDK from $customJdkPath")
        }
    } ?: logger.warn("Unknown property '$jdkAlias'")
}

project(":app").setupCustomJdk("jdk-11")
project(":lib").setupCustomJdk("jdk-8")