ci/docpush: add missing dependency (#264)

Summary:
This adds the missing `pandoc` dependency to the docpush CI step.

Pull Request resolved: #264

Test Plan:
Test with docpush manually enabled on the PR
https://github.com/pytorch/torchx/pull/264/checks?check_run_id=3908761230

Reviewed By: aivanou

Differential Revision: D31692193

Pulled By: d4l3k

fbshipit-source-id: 0fcb9b5667ec096d458d4e293c0cd1b34d402f7d

Author: d4l3k

.github/workflows/doc-build.yaml

@@ -60,6 +60,10 @@ jobs:
           set -ex
           git config --global user.email "[email protected]"
           git config --global user.name "TorchX CI Runner"
+      - name: Install Dependencies
+        run: |
+          set -eux
+          sudo apt-get install -y pandoc
       - name: Build
         run: |
           set -ex