MULTIPLE_UPLOADS unbound variable Bug with post-command hook

[mgkeen]

Been getting this error for the last couple of days:

/var/lib/buildkite-agent/plugins/github-com-buildkite-plugins-artifacts-buildkite-plugin/hooks/post-command: line 61: MULTIPLE_UPLOADS: unbound variable

Nothing has changed in our pipeline. I see there was a commit recently that set a failure on unbound variables here:

0165d63#diff-1ec88eb02937eb86424b55abfa2769f0

This is in a pipeline step that only does an artifact download:

- artifacts:
          download: 
            from: "zips/*.zip"
            to: infrastructure/main

[mgkeen]

Raised a PR with what i believe is a fix