Windows: fix stage cleaning for long paths (#45786)

Paths over 260 characters in length are not handled by `shutil.rmtree`
unless they use the extended-length path syntax (using a prefix of
"\\?\").

This fixes an issue where stage cleaning fails when paths in a stage
exceed the normal 260-character limit.

This indicates that other parts of the codebase should be examined/
refactored to handle long paths.

lib/spack/llnl/util/filesystem.py

@@ -1624,6 +1624,12 @@ def remove_linked_tree(path):
             shutil.rmtree(os.path.realpath(path), **kwargs)
             os.unlink(path)
         else:
+            if sys.platform == "win32":
+                # Adding this prefix allows shutil to remove long paths on windows
+                # https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry
+                long_path_pfx = "\\\\?\\"
+                if not path.startswith(long_path_pfx):
+                    path = long_path_pfx + path
             shutil.rmtree(path, **kwargs)