Do not run update-github-pages.yml in forked repositories

[yutaro-sakamoto]

It is not necessary to run update-github-pages.yml of forked repositories since the purpose of the workflow is publishing official documents of opensource COBOL 4J.
We should modify update-github-pages.yml in order to run the workflow only if its repository is opensourcecobol/opensourcecobol4j.

[tsh-hashimoto]

Thanks for assigning me (and publishing this issue).

It seems to be able to solve this problem to apply the method described on the following page.
https://docs.github.com/ja/actions/writing-workflows/choosing-when-your-workflow-runs/using-conditions-to-control-job-execution

I try this.