ci: doc-publish: fix run condition syntax

To restrict the job to the upstream repository a new `if` entry was
added, however, another `if` was already present (violates yaml).
Combined the conditions into a single expression.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

.github/workflows/doc-publish.yml

@@ -19,8 +19,9 @@ jobs:
   doc-publish:
     name: Publish Documentation
     runs-on: ubuntu-latest
-    if: ${{ github.event.workflow_run.conclusion == 'success' }}
-    if: github.repository == 'zephyrproject-rtos/zephyr'
+    if: |
+      github.event.workflow_run.conclusion == 'success' &&
+      github.repository == 'zephyrproject-rtos/zephyr'
 
     steps:
     - name: Download artifacts