Fix shell syntax in version_git.sh and remove warning. (#43196)

Author: fredrikekre

base/version_git.sh

@@ -85,11 +85,10 @@ if [ -z "$fork_master_timestamp" ]; then
 fi
 
 build_system_directory="../.buildkite"
-if [[ -d "${build_system_directory}/.git" ]]; then
+if [ -d "${build_system_directory}/.git" ]; then
     build_system_commit=$(git -C "${build_system_directory}" rev-parse HEAD)
     build_system_commit_short=$(git -C "${build_system_directory}" rev-parse --short HEAD)
 else
-    echo "Warning: The build system directory does not exist or is not a Git repo: ${build_system_directory}" >&2
     build_system_commit=""
     build_system_commit_short=""
 fi