ci: bound the apt install and prefer the canonical Ubuntu archive

.github/workflows/db-backup.yml

@@ -91,8 +91,41 @@ jobs:
           version: 2.75.0
 
       - name: Install age
+        # The dump, encrypt, and upload step is the recovery point and needs the
+        # job budget. Without a step bound, one hung apt call consumes all 30
+        # minutes and no backup is taken. `age` is a single small package, so 5
+        # minutes is a wide margin over its normal install and it leaves 25 of
+        # the 30 job minutes for the backup itself.
+        timeout-minutes: 5
         run: |
-          sudo apt-get update -qq
+          set -uo pipefail
+          # The hosted runner resolves its Ubuntu mirror through
+          # /etc/apt/apt-mirrors.txt, which points at azure.archive.ubuntu.com.
+          # That mirror fails intermittently, and each failure costs minutes of
+          # apt retries. Prefer the canonical archive. Best-effort: an absent or
+          # already-canonical file changes nothing.
+          #
+          # The canonical archive is the source azure.archive.ubuntu.com mirrors,
+          # so the same suite resolves the same `age` package. The package name
+          # and its flags below are unchanged, so the encryption tool this backup
+          # depends on is the same build it has always been.
+          sudo sed -i \
+            's|http://azure.archive.ubuntu.com/ubuntu|http://archive.ubuntu.com/ubuntu|g' \
+            /etc/apt/apt-mirrors.txt 2>/dev/null || true
+          update_ok=""
+          for attempt in 1 2 3; do
+            if sudo apt-get update -qq; then
+              update_ok=1
+              break
+            fi
+            echo "::warning::apt-get update failed (attempt ${attempt}/3); retrying"
+            sleep $((attempt * 10))
+          done
+          if [ -z "$update_ok" ]; then
+            echo "::error::apt-get update failed three times; the Ubuntu mirror is unreachable"
+            exit 1
+          fi
+          set -e
           sudo apt-get install -y -qq age
 
       - name: Validate the exact source, private target, and recipient