Build: Fix repos.mavenLocal casing (#29289)

The sysprop repos.mavenLocal may be used to add the local .m2 maven
repository for testing snapshots of locally build dependencies.
Unfortunately this has to be checked in two different places (they cannot
be shared, due to buildSrc being built essentially as a separate
project), and the casing of the string sysprop lookups did not align.
This commit fixes BuildPlugin's checking of repos.mavenLocal to use the
correct casing (camelCase, to match the gradle dsl element).

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

@@ -311,8 +311,8 @@ class BuildPlugin implements Plugin<Project> {
     /** Adds repositories used by ES dependencies */
     static void configureRepositories(Project project) {
         RepositoryHandler repos = project.repositories
-        if (System.getProperty("repos.mavenlocal") != null) {
-            // with -Drepos.mavenlocal=true we can force checking the local .m2 repo which is
+        if (System.getProperty("repos.mavenLocal") != null) {
+            // with -Drepos.mavenLocal=true we can force checking the local .m2 repo which is
             // useful for development ie. bwc tests where we install stuff in the local repository
             // such that we don't have to pass hardcoded files to gradle
             repos.mavenLocal()