removed the usage of install-cva6.sh and add setup-env.sh

Created by: valentinThomazic

Related to : https://github.com/openhwgroup/cva6/issues/1727

Removed verif/regress/install-cva6.sh which was installing spike, verilator but also setting the env variables for simulation despite its name. Removed verif/regress/install-riscv-dv.shsince it was also setting sim env variables and trying to install python packages each time it was run.

Instead, verif/regress/install-verilator.sh and verif/regress/install-spike.sh are directly used to install verilator and spike and a new file verif/sim/setup-env.sh sets the env variables for simulations.

Removed the ("broken" see https://github.com/openhwgroup/cva6/issues/1737) install skip mechanism in the process, which was only used once in gitlab-ci.yml. Instead, the installation scripts are simply not run.

Updated the README accordingly.