Bodhi compose config: fix templating in 'repos' definition
    
    You can't nest jinja's templating markers like this, it turns
    out, they just wind up as literal strings. You also can't use
    f-strings, sadly (tried it locally, it errors out). We have to
    use older-style `.format()`, that works. I went with a fairly
    explicit syntax to make it clear what's going on.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>