ci: fix host dependencies install step

Seems like ubuntu-slim is stripped away of some apt cache to make it
even slimmer than ubuntu-latest

Author: thunder-coding

.github/workflows/build.yml

@@ -14,6 +14,7 @@ jobs:
 
       - name: Install host dependencies
         run: |
+          sudo apt-get update
           sudo apt-get install pandoc
 
       - name: Prepare