fix(build): make the manifest retry cleanup work under sh

[edbfi]

Problem

The on_retry_command of Create Manifests always fails. nick-fields/retry executes it with /bin/sh (it uses execSync without a shell; the shell input only applies to command), and the value has no |, so YAML folds the loop onto one line:

while read manifest; do docker manifest rm $(awk '{print $1}' <<< ${manifest}) || true done < manifest.list

dash rejects both the here-string and the missing ; before done:

/bin/sh: 1: Syntax error: redirection unexpected

The manifest lists created before the failure stay in place, so the retries hit refusing to amend an existing manifest list with no --amend flag and a transient registry error can fail the job.

Seen in my edbfi/sabnzbd run 37594116659, attempt 1, job publish (ghcr.io).

First try: five lists were created, then two docker manifest create calls hit ghcr.io client timeouts and the cleanup failed with the error above.
Second try: refusing to amend for those five, plus two more timeouts.
Third try: ghcr.io responded again, but six of the seven lists were refused, so the job failed.

Fix

-          on_retry_command:
+          on_retry_command: |
             while read manifest; do
-              docker manifest rm $(awk '{print $1}' <<< ${manifest}) || true
+              docker manifest rm "${manifest%% *}" || true
             done < manifest.list

${manifest%% *} is the first field of each manifest.list line, which is what the awk call extracted. The fixed block passes dash -n.