Seems like the first failure and possibly a flake.

log snippet

chromite.lib.cros_build_lib.RunCommandError: return code: 1; command: git push cros prebuilt_branch:refs/heads/master
remote: 
remote: Processing changes: refs: 3
remote: Processing changes: refs: 3, done            
To https://chromium.googlesource.com/a/chromiumos/overlays/board-overlays
 ! [remote rejected]       prebuilt_branch -> master (failed to lock)
error: failed to push some refs to 'https://chromium.googlesource.com/a/chromiumos/overlays/board-overlays'
[W git.go:283] Transient error string identified in STDERR: " ! [remote rejected]       prebuilt_branch -> master (failed to lock)\n"
[W git.go:294] Retrying after 3s (rc=1): transient error string encountered
To https://chromium.googlesource.com/a/chromiumos/overlays/board-overlays
 ! [rejected]              prebuilt_branch -> master (fetch first)
error: failed to push some refs to 'https://chromium.googlesource.com/a/chromiumos/overlays/board-overlays'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.
[W git.go:317] Command completed with rc 1 after 1 transient failure(s).



Failing build

https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8931332141189207888