Fix PublishingPlugin URI.create() bug on Windows

Author: dconeybe

buildSrc/src/main/java/com/google/firebase/gradle/plugins/publish/PublishingPlugin.java

@@ -121,7 +121,7 @@ public void apply(Project project) {
                                   repo.setUrl(
                                       URI.create(
                                           "file://"
-                                              + sub.getRootProject().getBuildDir()
+                                              + sub.getRootProject().getBuildDir().toString().replace('\\', '/')
                                               + "/m2repository"));
                                   repo.setName("BuildDir");
                                 }));