ci: apt times out and retries, and the install step stops after 10 minutes

A runner's package mirror stopped answering during apt-get update three
times on one push, leaving jobs waiting toward the 6-hour job limit.

.github/workflows/test.yml

@@ -58,10 +58,14 @@ jobs:
             dev/requirements.txt
 
       # Build headers for the sdist-only bindings (libvirt-python, python-ldap)
+      # A runner's package mirror sometimes stops answering: time out and
+      # retry instead of hanging until the job's 6-hour limit
       - name: Install System Dependencies
+        timeout-minutes: 10
         run: |
-          sudo apt-get update -qq
-          sudo apt-get install -y --no-install-recommends \
+          APT="-o Acquire::Retries=3 -o Acquire::http::Timeout=30 -o Acquire::https::Timeout=30"
+          sudo apt-get $APT update -qq
+          sudo apt-get $APT install -y --no-install-recommends \
             libvirt-dev \
             libldap2-dev \
             libsasl2-dev \