treesum: don't use dir_fd parameter in os.scandir()

This parameter was added in python 3.7, but we're only depending on 3.6
for RHEL.

Pass a path to `/proc/self/fd/...` instead.

osbuild/treesum.py

@@ -19,7 +19,7 @@ def treesum(m, dir_fd):
     The file, symlink and directory names and contents are recursively
     hashed, together with security-relevant metadata."""
 
-    with os.scandir(dir_fd) as it:
+    with os.scandir(f"/proc/self/fd/{dir_fd}") as it:
         for dirent in sorted(it, key=(lambda d: d.name)):
             stat_result = dirent.stat(follow_symlinks=False)
             metadata = {}