Switch Verilator testing to use v5.008 instead of v4.110

Created by: zchamski

This PR introduces support for Verilator v5 (v5.008 at the time of PR creation). The main modifications are:

• A change in the use of shell variables to configure Verilator:
• In Verilator v5, VERILATOR_ROOT is required to point to the root of the source tree of Verilator
• A separate variable (currently named VERILATOR_INSTALL_DIR) is needed to designate the installation directory of Verilator
• If not defined explicitly by the user, VERILATOR_INSTALL_DIR is set to the parent directory of $VERILATOR_ROOT.
• Path settings for Verilator-related executables and include files are expressed using $VERILATOR_INSTALL_DIR.
• A change in the build flow of Verilator: instead of downloading and untarring a source package tarball, the source code is cloned from Verilator Github repository.
• A patch is applied to the source code of Verilator (it will be upstreamed shortly.)
• FORNOW, the cva6/sim/Makefile rule for Verilator invocation passes the --no-timing Verilator option as part of the value of Makefile variable verilator.