fix: skip test-sdist job on doc-only PRs

Match existing test jobs' doc-only guard so docs-only PRs don't
run source-build validation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

.github/workflows/ci.yml

@@ -138,7 +138,7 @@ jobs:
       - ci-vars
       - should-skip
     name: Test sdist
-    if: ${{ github.repository_owner == 'nvidia' && !fromJSON(needs.should-skip.outputs.skip) }}
+    if: ${{ github.repository_owner == 'nvidia' && !fromJSON(needs.should-skip.outputs.skip) && !fromJSON(needs.should-skip.outputs.doc-only) }}
     secrets: inherit
     uses: ./.github/workflows/test-sdist.yml
     with: