how to set vimrc tags to use shell environment variables

I tried but failed and need an expert help. In a .vimrc I have the following:

set tags=/sandbox/myNameIsSam/tags

This works just fine. Inside gvim, I can load the tags file and everything is awesome. However, I would like for each user to have their own project tags file. How and why is not important here. The below setting doesn't seem to do what I need. Can someone help?

let projectTagFile='/sandbox/'.$USERNAME.'/tags'
set tags=projectTagFile