SCons: Fix potential error when pruning cache on CI

This could cause spurious errors on CI when trying to prune the cache,
as for some reason it tries to remove files/paths which do not exist.

That points at a bug in the `cache_progress` logic but at least this
workaround should prevent CI failures.

methods.py

@@ -934,9 +934,12 @@ def show_progress(env):
 
     def progress_finish(target, source, env):
         nonlocal node_count, progressor
+        try:
             with open(node_count_fname, "w") as f:
                 f.write("%d\n" % node_count)
             progressor.delete(progressor.file_list())
+        except Exception:
+            pass
 
     try:
         with open(node_count_fname) as f: