Set `next-error-last-buffer' after running sbt command

Hi,

I noticed that when I have a sbt buffer opened with compilation errors, using `next-error` (M-g M-n) does not jump to the sbt error but uses the previous compilation buffer, e.g. `grep` instead.  I am not sure if this is the correct way to do it but it at least fixes the behavior described above.

You can reproduce it like this:

- M-x grep, search for something with matches and goto some match
- Open scala file
- Run sbt and get errors
- M-g M-n will jump to the grep match, not the sbt error

Let me know what you think.

Best,
Markus