Replace deprecated `set-output` command with environment file

[jongwooo]

Describe the issue In workflow, set-output command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information, see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

[jessebot]

This code base is now in: https://github.com/argoproj/argo-cd/tree/master/applicationset I made the same mistake recently :)