Multi-arch upgrade tests fail to handle parent build with missing arch

[jlebon]

Our bump-lockfile job hit the following when running the aarch64 upgrade test:

[2022-07-28T21:04:40.149Z] + set -xeuo pipefail
[2022-07-28T21:04:40.149Z] ++ umask
[2022-07-28T21:04:40.149Z] + '[' 0000 = 0000 ']'
[2022-07-28T21:04:40.149Z] + umask 0022
[2022-07-28T21:04:40.149Z] + cosa kola --rerun --upgrades --no-test-exit-error
[2022-07-28T21:04:41.068Z] kola -p qemu-unpriv --output-dir tmp/kola-upgrade run-upgrade --rerun --no-test-exit-error --qemu-image-dir tmp/kola-qemu-cache -v --find-parent-image
[2022-07-28T21:04:41.068Z] 2022-07-28T21:04:40Z cli: Started logging at level INFO
[2022-07-28T21:04:41.068Z] 2022-07-28T21:04:40Z cli: Started logging at level INFO
[2022-07-28T21:04:41.323Z] Error: Received a 403 error in http response for: https://builds.coreos.fedoraproject.org/prod/streams/testing-devel/builds/36.20220727.20.1/aarch64/meta.json
[2022-07-28T21:04:41.323Z] 2022-07-28T21:04:41Z cli: Received a 403 error in http response for: https://builds.coreos.fedoraproject.org/prod/streams/testing-devel/builds/36.20220727.20.1/aarch64/meta.json
[2022-07-28T21:04:41.323Z] error: failed to execute cmd-kola: exit status 1
script returned exit code 1

The reason it failed is that aarch64 artifacts don't exist for this build.

We should do something similar to the pipeline here to find the parent commit: https://github.com/coreos/fedora-coreos-pipeline/blob/fc3532704a0c3082810d34ca461d52747a53150a/jobs/build.Jenkinsfile#L186-L203.

The kola area of interest is here: https://github.com/coreos/coreos-assembler/blob/73bdf387a9d25b461f718c76fa3455902acfddfb/mantle/cmd/kola/kola.go#L620-L639

(The comment there needs to be updated; bump-lockfile also falls in this category.) Instead of using the last release, it'd do the equivalent of the pipeline Groovy code to keep going backwards.