fix(ci): restrict deploy workflow to push on default branch to avoid issues

[rbadillap]

Summary

Updated the deploy workflow trigger to run only on push events to the default branch instead of pull_request.
This prevents OIDC token and permission errors that occur when the workflow executes from forked repositories.

List of notable changes:

• updated the workflow trigger to:

  on:
    push:
      branches: [$default-branch]

Steps to test:

1. Go to my previous PR, and open the Checks tab
2. You'll notice the .github/workflows/deploy_docs_preview_forks.yml passes, but the .github/workflows/deploy_docs_preview.yml fails.
3. I had the initiative to contribute on this fix since Rez left a comment on my PR pointing out the need to fix this issue

Reviewer checklist:

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

Screenshots:

Please try to provide before and after screenshots or videos

Before	After
	Now the Deploy workflow is executed only when a push is done on the default-branch. Means, the workflow is not getting triggered on PRs.

Additional Note

• This PR follows exactly the same approach as the actions/starter-workflow, to deploy on the specified event.

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Latest commit: 7d90d00

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Pull Request Overview

This PR fixes a CI/CD workflow issue by restricting the deploy workflow to only run on pushes to the default branch, preventing OIDC token and permission errors that occur when the workflow runs from forked repositories during pull requests.

• Changed workflow trigger from pull_request to push events on the default branch
• Added explanatory comment about the trigger change
• Addresses permission issues identified in a previous PR review