Build: Add elastic maven to repos used by BuildPlugin (#32549)

This commit adds the elastic repo, alongside maven central, to any
plugin using BuildPlugin. This is necessary now because the default
distribution, which the test framework uses by default, is now only
hosted on elastic maven. While inside the elasticsearch build this does
not matter, those that build external plugins with our build-tools could
have tests fail to find the distribution dependency.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -405,6 +405,10 @@ class BuildPlugin implements Plugin<Project> {
             repos.mavenLocal()
         }
         repos.mavenCentral()
+        repos.maven {
+            name "elastic"
+            url "https://artifacts.elastic.co/maven"
+        }
         String luceneVersion = VersionProperties.lucene
         if (luceneVersion.contains('-snapshot')) {
             // extract the revision number from the version with a regex matcher