Refactor: improve shell scripts

[namejsjeongkr]

• These changes aim to make the scripts more robust, easier to maintain, and more efficient while ensuring they follow the project's established coding patterns. (Please refer to the below key changes.)

• e2e-k8s.sh
  Enhanced error handling by adding set -o pipefail.
  Improved code structure by extracting repeated tasks into functions.
  Added informative log messages for better execution tracking.
  Standardized variable usage to ${VARIABLE} format for consistency.
  Retained temporary directory creation and cleanup logic for better resource management.

• push-latest-cli.sh
  Updated error handling to use set -o errexit -o nounset -o pipefail.
  Introduced an upload_file function to reduce code duplication.
  Implemented use of gsutil -m cp for parallel uploads to improve performance.
  Added log messages at key steps for clearer execution tracking.
  Standardized variable declarations using double quotes.

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• ✅ login: namejsjeongkr / name: Jinsoo Jeong (ce1710d)

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