Synchronize accesses to ApplicationArchivesBuildItem#allArchives

Avoids race reported in
quarkusio#43138

Closes quarkusio#43138

core/deployment/src/main/java/io/quarkus/deployment/builditem/ApplicationArchivesBuildItem.java

@@ -42,7 +42,7 @@ public Collection<ApplicationArchive> getApplicationArchives() {
     /**
      * @return A set of all application archives, including the root archive
      */
-    public Set<ApplicationArchive> getAllApplicationArchives() {
+    synchronized public Set<ApplicationArchive> getAllApplicationArchives() {
         if (allArchives == null) {
             HashSet<ApplicationArchive> ret = new HashSet<>(applicationArchives);
             ret.add(root);