handle build with arch mismatch; support buildfetch --find-build-for-arch

[dustymabe]

commit a8896bc193774b2ead347e6ee75c89db187319b4
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Fri Jul 22 11:56:28 2022 -0400

    cmd-buildfetch: add --find-build-for-arch option
    
    This instructs buildfetch to traverse the builds.json history to
    find the most recent build for the given architecture.

commit 54cd68e0af7e796ac4cc4a30bcd1a43b6c45ac0a
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Fri Jul 22 11:55:42 2022 -0400

    cmd-buildfetch: bring the ostree ref delete into the loop
    
    There will be a ref per arch so we need to do it on each loop
    iteration.

commit 65e2d8b51c2bcc091803587967448347552aa435
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Fri Jul 22 11:54:40 2022 -0400

    cmd-buildfetch: continue loop if arch missing
    
    The user could have passed in multiple arches so we should continue
    the loop rather than returning.

commit 62064b21dbea936105efc48dbc95ea30147c8b30
Author: Dusty Mabe <dusty@dustymabe.com>
Date:   Fri Jul 22 11:45:26 2022 -0400

    cosalib/builds: add get_latest_for_arch function; use it in cmd-build
    
    Right now cmd-build will just pick up whatever the latest symlink points
    to and try to get previous build information from that, but what if
    the absolute latest build doesn't have a build for the architecture we
    are targetting? Currently cmd-build will fail with:
    
    ```
    /usr/lib/coreos-assembler/cmd-build: line 163: /srv/builds/36.20220720.20.5/aarch64/meta.json: No such file or directory
    error: failed to execute cmd-build: exit status 1
    ```
    
    What we really need is to get the latest build for the given
    architecture and use that in cmd-build instead.

[cgwalters]

Nicely split up changes!