fix: Use single '$' for ORIGIN parameter expansion

* Use of '$$' for the parameter expansion of ORIGIN results in
  a failure to properly resolve $ORIGIN and for LDFLAGS to be set
  improperly. Use '$ORIGIN' instead.

setup.py

@@ -82,7 +82,7 @@ def build_extensions(self):
                 f"--with-cgaldir={cgal_dir}",
                 "--enable-swig",
                 "--enable-pyext",
-                "LDFLAGS=-Wl,-rpath,$$ORIGIN/_fastjet_core/lib:$$ORIGIN",
+                "LDFLAGS=-Wl,-rpath,$ORIGIN/_fastjet_core/lib:$ORIGIN",
             ]
 
             try: