Replace deprecated command with environment file

[jongwooo]

Description

Closes #400

Update .github/workflows/update-images.yaml to use environment file instead of deprecated set-output command.
For more information, see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

I found the workflow file that use set-output command through the following command:

$ find . -name '*.yml' -o -name '*.yaml' | xargs egrep '\bset-output\b'

AS-IS

echo "::set-output name=modified::true"

TO-BE

echo "modified=true" >> $GITHUB_OUTPUT