execute: Pass AT_FDCWD instead of -1
    
    Let's enforce that callers pass AT_FDCWD as read_dfd to load_credential()
    to avoid an assert() in read_full_file_full() if read_dfd is -1.
    
    (cherry picked from commit 661e4251a5b157d1aee1df98fbd2f0c95285ebba)