[GH] Fix script injection vulnerability in workflow checkers

[himani2411]

Description of changes

• Use intermediate environment variables for github.event.pull_request.base.ref instead of inline ${{ }} expressions in run scripts, following GitHub's recommended mitigation for script injection attacks.

Reference: https://docs.github.com/en/actions/reference/security/secure-use#use-an-intermediate-environment-variable

References

• Link to impacted open issues.
• Link to related PRs in other packages (i.e. cookbook, node).
• Link to documentation useful to understand the changes.

Checklist

• Make sure you are pointing to the right branch.
• If you're creating a patch for a branch other than develop add the branch name as prefix in the PR title (e.g. [release-3.6]).
• Check all commits' messages are clear, describing what and why vs how.
• Make sure to have added unit tests or integration tests to cover the new/modified code.
• Check if documentation is impacted by this change.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.