Address a ShellCheck issue

Author: mokagio

.buildkite/shared-pipeline-vars

@@ -4,5 +4,6 @@ export IMAGE_ID="xcode-27.0-beta"
 export CI_TOOLKIT="automattic/a8c-ci-toolkit#5.7.0"
 export DOCKER_PLUGIN="docker#v5.13.0"
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-export RUBY_VERSION="$(cat "$SCRIPT_DIR/../.ruby-version")"
+RUBY_VERSION="$(cat "$SCRIPT_DIR/../.ruby-version")"
+export RUBY_VERSION
 export DOCKER_RUBY_IMAGE="public.ecr.aws/docker/library/ruby:$RUBY_VERSION"