SR-237 calls for `build-script` and `build-script-impl` to be merged. This
commit takes another step towards that goal by moving the logic that finds
the path to the CMake executable up into `build-script`.
Users of `build-script` were previously able to specify which CMake
they wished to use via `build-script-impl` args, like so:
```
$ utils/build-script -- --cmake=/foo/bar/cmake
```
This commit preserves that behavior, while also allowing users to
specify that path via `build-script` args:
```
$ utils/build-script --cmake=/baz/cmake -- --other --flags
```
