[bb] add .gitignore to prevent accidental uploads of the `bb` binary.
[bb] add .gitignore to prevent accidental uploads of the `bb` binary.

R=nodir

Change-Id: I86066c03974e8982f129bb1b0028d894e0d22814
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2912095
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go