Step "Save previous sitemap file" doesn't actualy save the previous s…

…itemap

Checkout gh-pages branch and update the "Save previous sitemap file" step to
save the sitemap.xml file in that checked out branch to /tmp/previous-sitemap.xml.
Rename the exiting step "Checkout gh-pages source" to "Update gh-pages source"
to better describe its purpose: to retrieve the latest published sitemap.xml.

.github/workflows/github-pages.yml

@@ -16,16 +16,21 @@ jobs:
       - name: Copy CNAME file to ./blog
         run: cp -np CNAME ./blog
         shell: bash
+      - name: Checkout gh-pages source
+        uses: actions/checkout@v2
+        with:
+          ref: "gh-pages"
+          path: "BuiltGitHubPages"
       - name: Save previous sitemap file
-        run: "[ -f ./blog/_site/sitemap.xml ] && cp -np ./blog/_site/sitemap.xml /tmp/previous-sitemap.xml || exit 0"
+        run: "[ -f ./BuiltGitHubPages/sitemap.xml ] && cp -np ./BuiltGitHubPages/sitemap.xml /tmp/previous-sitemap.xml || exit 0"
         shell: bash
       - name: Build and publish site
         uses: helaili/[email protected]
         with:
           jekyll_src: "./blog"
           target_branch: "gh-pages"
           token: ${{ secrets.GITHUB_TOKEN }}
-      - name: Checkout gh-pages source
+      - name: Update gh-pages source
         uses: actions/checkout@v2
         with:
           ref: "gh-pages"