ignore go workspace files

Created by: keegancsmith

This is a new feature in go1.18 to simplify working on multiple modules. For example I can easily test sourcegraph with local changes to zoekt by running the command:

go work init . ../../google/zoekt/

These files shouldn't be committed, as they are files that are specific to someones task.

Test Plan: my go.work file doesn't show up in git status