Remove the 'z' format specifier in the storage key names. (#14547)

Replace the 'z' format specifier added in #14253 with one that's supported in all platforms.

src/lib/support/DefaultStorageKeyAllocator.h

@@ -40,7 +40,11 @@ class DefaultStorageKeyAllocator
     // Access Control List
 
     const char * AccessControlList() { return Format("acl"); }
-    const char * AccessControlEntry(size_t index) { return Format("acl/%zx", index); }
+    const char * AccessControlEntry(size_t index)
+    {
+        // This cast will never overflow because the number of ACL entries will be low.
+        return Format("acl/%x", static_cast<unsigned int>(index));
+    }
 
     // Group Data Provider