Use robust path when reading .ruby-version (#680)

Author: mokagio

.buildkite/shared-pipeline-vars

@@ -3,5 +3,6 @@
 export IMAGE_ID="xcode-26.1.1"
 export CI_TOOLKIT="automattic/a8c-ci-toolkit#5.7.0"
 export DOCKER_PLUGIN="docker#v5.13.0"
-export RUBY_VERSION="3.2.2"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+export RUBY_VERSION="$(cat "$SCRIPT_DIR/../.ruby-version")"
 export DOCKER_RUBY_IMAGE="public.ecr.aws/docker/library/ruby:$RUBY_VERSION"