[scripts] change protobuf installation directory name

Summary: The protobuf repo has a file named BUILD in it, so install_protobuf.sh fails with the error, mkdir: build: File exists. This appears to only be an issue on macOS, perhaps because Debian is case sensitive.

Test Plan: All the Buildkite CI workflows should pass

Reviewers: jon, tomek, atul

Reviewed By: atul

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6522