Remove mavenLocal from default repositories

[sghill]

In order to better support reproducible builds
As a user
I want mavenLocal() to not be added by default

User Changes Required

Users who depend on mavenLocal() will need to add this declaration:

repositories {
    mavenLocal()
}

Context

The Gradle docs cover several reasons why mavenLocal() shouldn't be added and include the recommendation:

As a general advice, you should avoid adding mavenLocal() as a repository.

If jenkinsPlugin.configureRepositories = true (default), mavenLocal() is the first repository added. After building Jenkins from source, I started to get this failure:

* What went wrong:
    Execution failed for task ':runtime'.
    > Could not resolve all files for configuration ':runtimeClasspath'.
       > Could not find workflow-step-api-2.20.jar (org.jenkins-ci.plugins.workflow:workflow-step-api:2.20).
         Searched in the following locations:
                 file:~/.m2/repository/org/jenkins-ci/plugins/workflow/workflow-step-api/2.20/workflow-step-api-2.20.jar

This fails to search other repositories because order matters:

Once a repository returns a metadata result, following repositories are ignored.