stage.steal_source: preserve symlinks

This avoids dangling symlink errors. ignore_dangling_symlinks option would be more-targeted but is only available for Python >= 3.2 (#28318)

lib/spack/spack/stage.py

@@ -543,7 +543,7 @@ def steal_source(self, dest):
         for entry in hidden_entries + entries:
             if os.path.isdir(entry):
                 d = os.path.join(dest, os.path.basename(entry))
-                shutil.copytree(entry, d)
+                shutil.copytree(entry, d, symlinks=True)
             else:
                 shutil.copy2(entry, dest)