Koschei: disable builds on aarch64 for now
    
    aarch64 builders have signifficantly lower capacity that other arches.
    They are usually loaded with tasks, while ppc64le and x86_64 ones are
    mostly free.  This slows down Koschei as it does not submit scratch
    builds until load on aarch64 drops below 75 %.
    
    The following table illustrates capacity of Koji builders in default
    channel as of August 17, 2022:
    
        ┌─────────┬──────────┬──────────┐
        │    arch │ builders │ capacity │
        ├─────────┼──────────┼──────────┤
        │  x86_64 │       46 │      152 │
        │  armhfp │       28 │       56 │
        │ aarch64 │       34 │       68 │
        │   ppc64 │       37 │      148 │
        │ ppc64le │       37 │      148 │
        │   s390x │       27 │       67 │
        └─────────┴──────────┴──────────┘