Deconflict top level build directory and bazel BUILD file

[dtolnay]

Having both causes silly trouble on case insensitive filesystems. 🤦‍♀

% git status
HEAD detached at origin/master
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	deleted:    BUILD

no changes added to commit (use "git add" and/or "git commit -a")

% git checkout -- BUILD

% git status
HEAD detached at origin/master
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	deleted:    build/rust.bzl

no changes added to commit (use "git add" and/or "git commit -a")

% git checkout -- build/rust.bzl

% git status
HEAD detached at origin/master
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	deleted:    BUILD

no changes added to commit (use "git add" and/or "git commit -a")