When generating MSI and OSX packages we are inspecting the environment variable JENKINS_SSH_KEY and if present it's used for authentication of CLI commands that invoke the distfork plugin (see here).
The CLI command is then composed with a "-i /path/to/key" option. AFAIK now in the CLI this option does only make sense if used together with "-ssh" or "-remoting", being the latter not recommended.
So basically if the normal thing will be to use "-ssh", which requires specifying a user, we need to enable a way of doing this.
I propose using a new environment variable called JENKINS_SSH_USER in a similar way the JENKINS_SSH_KEY one is used. If it has some content, the -ssh -user email@example.com part will be added to the command.