Add mavenLocal project property for Gradle

Related to https://github.com/checkstyle/checkstyle/issues/7322

To be able to build the project against `mavenLocal()`, add a
`mavenLocal` project property which can be specified as command line arg
`-PmavenLocal`, so a `mavenLocal()` is added into `repositories`
conditionally

build.gradle

@@ -104,6 +104,9 @@ allprojects {
 
 	repositories {
 		maven { url 'https://repo.spring.io/libs-milestone' }
+		if (project.hasProperty('mavenLocal')) {
+			mavenLocal()
+		}
 		if (version.endsWith('BUILD-SNAPSHOT')) {
 			maven { url 'https://repo.spring.io/libs-snapshot' }
 			maven { url "https://oss.jfrog.org/artifactory/libs-snapshot" } // RSocket