CI: clarify implicit prereqs

Author: davidjb

.github/workflows/build.yml

@@ -39,6 +39,7 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install prerequisites
         run: |
+          sudo apt install -y git wget gcc cmake
           sudo apt install -y libpcre3-dev libssl-dev zlib1g-dev
       - name: Build
         env: