Makefile `r_build` target is sometimes used inappropriately

In the [Makefile](https://github.com/cmu-delphi/forecast-eval/blob/f12ab6f303ba81d6cbc32d61720061474496a00f/Makefile), the `r_build` target is listed as a dependency of the [`score_forecast` target](https://github.com/cmu-delphi/forecast-eval/blob/f12ab6f303ba81d6cbc32d61720061474496a00f/Makefile#L24) and the contained `docker run` call does use the `forecast-eval-build` image created in `r_build` -- great. The `start_dev` target, on the other hand, has `r_build` as a dependency but uses an entirely different image. It looks like the `r_build` dependency is completely extraneous.

Open the Makefile and inspect the start_dev and r_build targets, comparing start_dev's dependencies with the Docker image it uses. Confirm whether start_dev can run without invoking r_build, then verify that start_dev no longer triggers the unnecessary build step.

Written by the indexing model from the issue text.