Fix post-command (#43)

Author: Paul Balaji

.buildkite/hooks/post-command

@@ -3,6 +3,6 @@ set -euo pipefail
 [[ -n "${DEBUG:-}" ]] && set -x
 
 # Only run purge script on Windows agents, as `premerge` runs on a Linux box
-if [[ ${BUILDKITE_AGENT_META_DATA_OS} == "windows" ]]; then
+if [[ ${BUILDKITE_AGENT_META_DATA_PLATFORM} == "windows" ]]; then
     powershell -NoProfile -NonInteractive .shared-ci/scripts/purge.ps1 -projectRoot "$(pwd)/"
 fi