Don't force json config - allow openlane Makefile to work with both config.json and config.tcl files

[mattvenn]

I am currently trying to avoid the json config as it's not as full featured as the tcl one (and it's ugly/hard to read)

Example: (can't source files for example : https://github.com/The-OpenROAD-Project/OpenLane/issues/1538)

Please allow the caravel_user_project Makefile to support .tcl config files.

At the moment I have to change the makefile on line 64:
back to tcl for my designs, then change it back to .json for the top level hardening.