Fixed VERILATOR_ROOT variable extraction

[fadialzammar]

If the installed version of Verilator was built in a different environment, the existing command will extract the wrong VERILATOR_ROOT path. For example, when using the oss-cad-suite-build, verilator -V returns the following:

Verilator 5.027 devel rev v5.026-136-g48c71ef76

Copyright 2003-2024 by Wilson Snyder.  Verilator is free software; you can
redistribute it and/or modify the Verilator internals under the terms of
either the GNU Lesser General Public License Version 3 or the Perl Artistic
License Version 2.0.

See https://verilator.org for documentation

Summary of configuration:
  Compiled in defaults if not in environment:
    SYSTEMC            = 
    SYSTEMC_ARCH       = 
    SYSTEMC_INCLUDE    = 
    SYSTEMC_LIBDIR     = 
    VERILATOR_ROOT     = /yosyshq/share/verilator
    SystemC system-wide = 0

Environment:
    MAKE               = 
    PERL               = 
    PYTHON3            = 
    SYSTEMC            = 
    SYSTEMC_ARCH       = 
    SYSTEMC_INCLUDE    = 
    SYSTEMC_LIBDIR     = 
    VERILATOR_BIN      = 
    VERILATOR_ROOT     = /opt/oss-cad-suite/share/verilator

Supported features (compiled-in or forced by environment):
    COROUTINES         = 1
    SYSTEMC            = 

The existing command returns /yosyshq/share/verilator, the path used when built by YosysHQ, rather than /opt/oss-cad-suite/share/verilator, the path Verilator was installed to on the current system. Using the --getenv flag instead correctly returns the second path.

[ZipCPU]

Sigh. I keep ending up going back and forth on this issue. One fix after another seems to break whatever it is I'm doing.

[ZipCPU]

... the ?= was supposed to solve this issue in the first place ...