Update bazel 2.0.0 --> 2.1.0

[aherrmann]

No description provided.

[aherrmann]

It looks like repository rules became a little more lazy with the Bazel update. The stack_update mechanism used to prevent a race on the Hackage lock stopped working on Windows. Adding a read of the stack_update file was sufficient to enforce the dependency again. Long-term I'd like to find a better solution, ideally some form of pinning similar to rules_jvm_external. But that's out of scope for this PR, I'll add a separate issue about that idea.