If you cloned the repository as it is and didn't move things around, it should work fine.
You should see-
<local-path>/hdl_master/library/axi_hdmi_tx/axi_hdmi_tx_hw.tcl
--- and so forth for other libraries ----
<local-path>/hdl_master/projects/ad_fmcjesdadc1/zc706/system_project.tcl
Assuming this, and you haven't set that environment variable, things should work fine.
If it is not and you moved around the library, you can set that environment variable.
I would discourage this unless you have a library structure that is different from this and want to keep things your way.