Use explicit comparison in if condition of github action (#1821)

Author: antonwolfy

.github/workflows/build-sphinx.yml

@@ -155,7 +155,7 @@ jobs:
       # https://github.com/marketplace/actions/github-pages-action
       # The step is only used to build docs while pushing a PR to "master"
       - name: Deploy docs
-        if: env.GH_EVENT_PUSH_UPSTREAM
+        if: env.GH_EVENT_PUSH_UPSTREAM == 'true'
         uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
@@ -168,7 +168,7 @@ jobs:
 
       # The step is only used to build docs while pushing to PR branch
       - name: Publish pull-request docs
-        if: env.GH_EVENT_OPEN_PR_UPSTREAM
+        if: env.GH_EVENT_OPEN_PR_UPSTREAM == 'true'
         uses: peaceiris/actions-gh-pages@4f9cc6602d3f66b9c108549d475ec49e8ef4d45e # v4.0.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
@@ -183,7 +183,7 @@ jobs:
 
       # The step is only used to build docs while pushing to PR branch
       - name: Comment with URL to published pull-request docs
-        if: env.GH_EVENT_OPEN_PR_UPSTREAM
+        if: env.GH_EVENT_OPEN_PR_UPSTREAM == 'true'
         env:
           PR_NUM: ${{ github.event.number }}
         uses: mshick/add-pr-comment@b8f338c590a895d50bcbfa6c5859251edc8952fc # v2.8.2