Fix leading / during spack buildcache -f ... (#24028)

For me the buildcache force overwrite option does not work. It tries to
delete a file, but errors with a key error, apparently because the
leading / has to be removed.

lib/spack/spack/util/web.py

@@ -277,7 +277,7 @@ def remove_url(url, recursive=False):
                 r = s3.delete_objects(Bucket=bucket, Delete=delete_request)
                 _debug_print_delete_results(r)
         else:
-            s3.delete_object(Bucket=bucket, Key=url.path)
+            s3.delete_object(Bucket=bucket, Key=url.path.lstrip('/'))
         return
 
     # Don't even try for other URL schemes.