binary_distribution: Do not look in sub-mirrors when indexing (#50389)

When indexing top level specs, eg, in s3://spack-binaries/develop,
do not sync manifests from all the stacks.  Instead, add the path
to the spec manifests to the url to sync, so that only items in
s3://spack-binaries/develop/v3/manifests/spec are copied to the
local system.

lib/spack/spack/binary_distribution.py

@@ -754,6 +754,7 @@ def file_read_method(manifest_path):
         cache_entry.destroy()
         return spec_dict
 
+    url_to_list = url_util.join(url, buildcache_relative_specs_url())
     sync_command_args = [
         "s3",
         "sync",
@@ -761,11 +762,11 @@ def file_read_method(manifest_path):
         "*",
         "--include",
         "*.spec.manifest.json",
-        url,
+        url_to_list,
         tmpspecsdir,
     ]
 
-    tty.debug(f"Using aws s3 sync to download manifests from {url} to {tmpspecsdir}")
+    tty.debug(f"Using aws s3 sync to download manifests from {url_to_list} to {tmpspecsdir}")
 
     try:
         aws(*sync_command_args, output=os.devnull, error=os.devnull)