Use the appropriate function to remove files in the stage directory (#29690)

We shouldn't be using "remove_linked_tree" to remove the lock file,
since that function expects to receive a directory path as an
argument.

Also, as a further measure to avoid regression, this commit restores
the "ignore_errors=True" argument on linux and adds a unit test
checking that "remove_linked_tree" doesn't change file permissions
as a side effect of a failure to remove.

lib/spack/llnl/util/filesystem.py

@@ -27,7 +27,7 @@
 from spack.util.executable import Executable
 from spack.util.path import path_to_os_path, system_path_filter
 
-is_windows  = _platform == 'win32'
+is_windows = _platform == 'win32'
 
 if not is_windows:
     import grp
@@ -1201,12 +1201,16 @@ def onerror(func, path, exe_info):
             tty.warn(e)
             pass
 
+    kwargs = {'ignore_errors': True}
+    if is_windows:
+        kwargs = {'onerror': onerror}
+
     if os.path.exists(path):
         if os.path.islink(path):
-            shutil.rmtree(os.path.realpath(path), onerror=onerror)
+            shutil.rmtree(os.path.realpath(path), **kwargs)
             os.unlink(path)
         else:
-            shutil.rmtree(path, onerror=onerror)
+            shutil.rmtree(path, **kwargs)
 
 
 @contextmanager