sim_if/modelsim: use vmap to find modelsim.ini

[bjornfor]

• This creates a single source of truth for finding modelsim.ini. It is now possible to use MODELSIM=/path/to/custom/modelsim.ini and both ModelSim and vunit will use that. (The VUNIT_MODELSIM_INI environment variable can still be used to override modelsim.ini only for vunit.)
• This fixes detecting modelsim.ini in the quartus-prime-lite package from https://github.com/nixos/nixpkgs, which exposes binaries at $prefix/bin, but has no $prefix/modelsim.ini, because the latter would be an FHS violation.