ostree: Always substitute basearch
    
    When ref is not modified via pungi config, we read it from the treefile
    and substitute in basearch.
    
    When pungi is configured to replace it, it modifies the treefile and
    then used the value from config to avoid parsing the file. This however
    did not substitute the basearch value.
    
    We can simply use one code path for getting the value. This will work
    for both cases.
    
    Fixes: https://pagure.io/pungi/issue/866
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>