Fix: Wrap apt calls into a retry so we can ideally jump from a slow mirror to a faster one

.github/actions/apt-install/action.yml

@@ -0,0 +1,53 @@
+name: Install apt packages
+description: >
+  Installs apt packages with a per-attempt timeout and retries. The hosted runner's Azure apt mirror intermittently serves archives at a crawl, which apt's own timeout does not catch. A stuck attempt is killed and retried, which usually lands on a healthy mirror node.
+
+inputs:
+  packages:
+    description: Space separated list of packages to install
+    required: true
+  attempts:
+    description: Maximum number of attempts
+    default: "3"
+  attempt-timeout:
+    description: Seconds before a single attempt is killed
+    default: "180"
+runs:
+  using: composite
+  steps:
+    - name: Install ${{ inputs.packages }}
+      shell: bash
+      env:
+        PACKAGES: ${{ inputs.packages }}
+        ATTEMPTS: ${{ inputs.attempts }}
+        ATTEMPT_TIMEOUT: ${{ inputs.attempt-timeout }}
+      run: |
+        SUDO=""
+        if [ "$(id -u)" -ne 0 ]; then
+          SUDO="sudo"
+        fi
+
+        attempt_install() {
+          $SUDO apt-get update -q
+          # Intentionally unquoted, this is a list of package names
+          $SUDO apt-get install -q --yes --no-install-recommends $PACKAGES
+        }
+        export -f attempt_install
+        export SUDO PACKAGES
+
+        for attempt in $(seq 1 "${ATTEMPTS}"); do
+          echo "::group::apt install attempt ${attempt}/${ATTEMPTS}"
+          start=$(date +%s)
+          if timeout --kill-after=10 "${ATTEMPT_TIMEOUT}" bash -c attempt_install; then
+            echo "::endgroup::"
+            echo "Installed in $(( $(date +%s) - start ))s"
+            exit 0
+          fi
+          echo "::endgroup::"
+          echo "::warning::apt install attempt ${attempt}/${ATTEMPTS} failed or exceeded ${ATTEMPT_TIMEOUT}s"
+          # A killed attempt can leave dpkg half configured
+          $SUDO dpkg --configure -a || true
+        done
+
+        echo "::error::apt install failed after ${ATTEMPTS} attempts"
+        exit 1