chore: fix short-circuit of post_build step (#18374)

The buildspec was recently altered (in #18365) to skip the `POST_BUILD`/pack
step if the build failed; however, the command used was bash-specific and not
compliant with CodeBuild. Rewrite the check to be sh-compliant.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: njlynch

buildspec.yaml

@@ -28,7 +28,7 @@ phases:
   post_build:
     commands:
       # Short-circuit: Don't run pack if the above build failed.
-      - '[[ "$CODEBUILD_BUILD_SUCCEEDING" -eq 1 ]] || exit 1'
+      - '[ ${CODEBUILD_BUILD_SUCCEEDING:-0} -eq 1 ] || exit 1'
       - "[ -f .BUILD_COMPLETED ] && /bin/bash ./pack.sh"
       - /bin/bash ./scripts/cache-store.sh
 artifacts: