Hi Ashley,
Thanks very much for attaching the screenshots; that was a big help. In this case, since you're trying to set the value of a GP tool parameter, you can pass in the value that you want as a step argument. And since the value needs to be user-specific, I would recommend using a token or some combination of tokens to do so.
The "[SYS:CUR_JTX_USER]" token might be one option. It may also be possible to use an environment variable to help you out. (One way to see what's available is to open a command prompt in Windows and run the "set" command with no arguments.) On my machine, it looks like I may be able to build a path similar to the one that you need using the USERPROFILE environment variable. I can access this in a Workflow Manager token as, "[ENV:USERPROFILE]".
I agree that this isn't ideal, but maybe it's enough to get you past this issue...?
Brian D.