ci: set fake Git user for deploying

[pastelmind]

This fixes the following CI error:

webpack 5.73.0 compiled with 3 warnings in 13406 ms
Author identity unknown

*** Please tell me who you are.

Run

  git config --global user.email "[email protected]"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: empty ident name (for <runner@fv-az74-828.rg1nhevy3x5uhivy2ou0ehszxf.jx.internal.cloudapp.net>) not allowed

Error: Process completed with exit code 1.

The errors were observed in the following runs:

• https://github.com/webtoon/psd/runs/6828152676?check_suite_focus=true
• https://github.com/webtoon/psd/runs/6349346363?check_suite_focus=true

To resolve this issue, this PR makes the CI script use a fake Git user name and email (github-actions-bot <[email protected]>) when publishing the demo page. This is based on the following tips:

• https://github.com/tschaub/gh-pages#deploying-with-github-actions
• How to use with GitHub Actions? tschaub/gh-pages#345 (comment)