buildman.yml: fixed `manpage` job (#5539)

```
Invalid workflow file: .github/workflows/buildman.yml#L42
You have an error in your yaml syntax on line 42
```

.github/workflows/buildman.yml

@@ -39,15 +39,14 @@ jobs:
           path: output
 
   manpage:
-    - runs-on: ubuntu-22.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v3
 
       - name: Install missing software on ubuntu
-          if: contains(matrix.image, 'ubuntu')
         run: |
-            apt-get update
-            apt-get install -y xsltproc docbook-xsl
+          sudo apt-get update
+          sudo apt-get install -y xsltproc docbook-xsl
 
       - name: build manpage
         run: |