Update actions-gh-pages to 3.7.3

See peaceiris/actions-gh-pages@bbdfb20
Try to update to fix the broken environment setting
command that has been deprecated by GitHub.

.github/workflows/build.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Push to compgeolab.github.io
         if: success() && github.event_name == 'push'
         # Don't use tags: https://julienrenaux.fr/2019/12/20/github-actions-security-risk/
-        uses: peaceiris/actions-gh-pages@8a36f3edfc5d1cbae6b09e6f5a7d7b19e5b7a73b
+        uses: peaceiris/actions-gh-pages@bbdfb200618d235585ad98e965f4aafc39b4c501
         with:
           deploy_key: ${{ secrets.ACTIONS_DEPLOY_KEY }}
           external_repository: compgeolab/compgeolab.github.io