Fix Preview Release Notes GH Action for forks

Use hardcoded values instead of repo variables because
variables are not available in the PRs created from forks.

Author: m1kola

.github/workflows/preview_release_notes.yml

@@ -29,8 +29,10 @@ jobs:
         id: generate_release_notes
         run: python -m scripts.release.release_notes -s $INITIAL_COMMIT_SHA -v $INITIAL_VERSION -o release_notes_tmp.md
         env:
-          INITIAL_COMMIT_SHA: ${{ vars.RELEASE_INITIAL_COMMIT_SHA }}
-          INITIAL_VERSION: ${{ vars.RELEASE_INITIAL_VERSION }}
+          # We can not use environments set via GitHub UI because they will
+          # not be available in the pull requests running from forks.
+          INITIAL_COMMIT_SHA: 9ed5f98fc70c5b3442f633d2393265fb8a2aba0c
+          INITIAL_VERSION: 1.3.0
       - name: Add disclaimer to release notes preview
         run: |
           echo -e "_:warning: (this preview might not be accurate if the PR is not rebased on current master branch)_\n" > release_notes_preview.md