integrations/harbor fix username variable

What does this MR do and why?

Habor robot account use the syntax robot$<project>+<username>. If your harbor integration a robot account, it is not possible to use hte $HARBOR_USERNAME varaible.

Also the documentation refers to the varaible $HARBOR_USER, which doesn't exist.

I can try to had a spec test if you want.

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Actually:

After the patch

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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