Specify buildfile for Visual Studio generator

[Ace314159]

This specifies a buildfile for the Visual Studio generator, so cmake will not always reconfigure.

Fixes #111

[cottsay]

I'm not certain that this will work 100% of the time when a Visual Studio generator is used, but this certainly addresses the most common case and will do no harm otherwise.

Thanks!

[FelixPe]

@cottsay Shouldn't there be a check, if generator is not None as well, as for Ninja?

I ran into:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/colcon_core/executor/__init__.py", line 91, in __call__
    rc = await self.task(*args, **kwargs)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/colcon_core/task/__init__.py", line 93, in __call__
    return await task_method(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/colcon_ros/task/ament_cmake/build.py", line 59, in build
    rc = await extension.build(
         ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/colcon_cmake/task/cmake/build.py", line 87, in build
    rc = await self._reconfigure(args, env)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/colcon_cmake/task/cmake/build.py", line 133, in _reconfigure
    buildfile = get_buildfile(cmake_cache)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3/dist-packages/colcon_cmake/task/cmake/__init__.py", line 114, in get_buildfile
    if 'Visual Studio' in generator:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument of type 'NoneType' is not iterable