GH-50917: [C++] Fix shellcheck errors in cpp/build-support/*-flatbuffers.sh - #50918

GH-50917: [C++] Fix shellcheck errors in cpp/build-support/*-flatbuffers.sh#50918

kou merged 2 commits into

apache:mainapache/arrow:mainfrom

hiroyuki-sato:topic/shellcheck-cpp-build-flatbuffershiroyuki-sato/arrow:topic/shellcheck-cpp-build-flatbuffersCopy head branch name to clipboard