Add VUNIT_VIVADO_PATH env variable

[LarsAsplund]

This issue was moved from VUnit/vunit#811.

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The vivado.py python script pulls the path to the vivado binary from either the user's path or from method argument, rather than from a env variable like the simulator paths. Modifying the run_vivado method under vivado.py to pull from an env variable would make it much easier to work with multiple local vivado installs on a machine, and unify this code with how VUnit specifies other binaries.

The run_vivado method might look something like this

def run_vivado(tcl_file_name, tcl_args=None, cwd=None, vivado_path=None):
  """
  Run tcl script in Vivado in batch mode.
  Note: the shell=True is important in windows where Vivado is just a bat file.
  """
  vivado = os.environ.get("VUNIT_VIVADO_PATH", None) if vivado_path is None else str(Path(vivado_path).resolve() / "bin" / "vivado")
  cmd = f"{vivado} -nojournal -nolog -notrace -mode batch -source {str(Path(tcl_file_name).resolve())}"
  if tcl_args is not None:
      cmd += " -tclargs " + " ".join([str(val) for val in tcl_args])

  print(cmd)
  check_call(cmd, cwd=cwd, shell=True)