ci: doc-publish-pr: Improve workflow security

Improve workflow security.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/doc-publish-pr.yml

@@ -30,8 +30,12 @@ jobs:
 
     - name: Load PR number
       if: steps.download-artifacts.outputs.found_artifact == 'true'
-      run: |
-        echo "PR_NUM=$(<pr_num/pr_num)" >> $GITHUB_ENV
+      uses: actions/github-script@v7
+      with:
+        script: |
+          let fs = require("fs");
+          let pr_number = Number(fs.readFileSync("./pr_num/pr_num"));
+          core.exportVariable("PR_NUM", pr_number);
 
     - name: Check PR number
       if: steps.download-artifacts.outputs.found_artifact == 'true'