use os.sep instead of null byte to pad replacement paths in binaries (#15338)

* use os.sep instead of null byte to pad replacement paths in binaries

* Prefix the pading os.sep's

lib/spack/spack/relocate.py

@@ -412,7 +412,8 @@ def replace(match):
         if padding < 0:
             return data
         return match.group().replace(old_dir.encode('utf-8'),
-                                     new_dir.encode('utf-8')) + b'\0' * padding
+                                     os.sep.encode('utf-8') * padding +
+                                     new_dir.encode('utf-8'))
 
     with open(path_name, 'rb+') as f:
         data = f.read()