When IdString refcounting was expensive, it made sense to pass it by const reference
instead of by value, to avoid refcount churn. Now that IdString is not refcounted,
it's slightly more efficient to pass it by value.
Fixes these build errors I'm getting locally with `tcl-devel-9.0.0-7.fc42.x86_64`.
I guess Tcl 9 broke this.
```
passes/cmds/sdc/sdc.cc:438:6: error: no matching function for call to 'Tcl_ListObjLength'
438 | if (Tcl_ListObjLength(interp, listObj, &listLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
passes/cmds/sdc/sdc.cc:446:8: error: no matching function for call to 'Tcl_ListObjLength'
446 | if (Tcl_ListObjLength(interp, subListObj, &subListLength) == TCL_OK) {
| ^~~~~~~~~~~~~~~~~
/usr/include/tclDecls.h:1788:13: note: candidate function not viable: no known conversion from 'int *' to 'Tcl_Size *' (aka 'long *') for 3rd argument
1788 | EXTERN int Tcl_ListObjLength(Tcl_Interp *interp,
| ^
1789 | Tcl_Obj *listPtr, Tcl_Size *lengthPtr);
| ~~~~~~~~~~~~~~~~~~~
```
This uses the environment variable `YOSYS_PLUGIN_PATH` to provide multiple colon-delimited search paths for native plugins in a similar manner to `PATH` for executables and `PYTHONPATH` for Python modules.
This addresses https://github.com/YosysHQ/yosys/issues/2545, allowing Yosys to be better packaged in non-FHS environments such as Nix.