SELECTED ANSWER
replied on December 7, 2017
As far as I know forms does not return the entry id from the store in repository task. I believe the reason It doesn't return the id is that the storage process is asynchronous so the id isn't available straight away.
That said you have some options but they generally aren't the cleanest.
1) You could save the form to a specific folder in your repository with all the metadata information required to send the email and rather then have you forms process kick off the workflow use a starting rule that triggers when a file with the right template is created.
2) You could again save the form to a specific folder and use the instance/submission id in either the metatdata or the entry name to uniquely identify the form. Then in the workflow on a loop you could search the repository for the form with the matching id. As soon as the form shows up in the repository the workflow will find it and continue on its way.
Option 1 is probably cleaner but it depends on how much of the forms data you require access to. If you only process a few fields then deff go with option 1.