ci: prefer the canonical Ubuntu archive for the paper TeX install (#374)

My PR #373 bounded this step at 10 minutes but did not address why it hangs.
Main then went red at 7860278: the Azure mirror was failing, apt spent the
whole budget retrying it, and the bound fired. The bound behaved correctly --
10 minutes instead of 3h11m -- but a red main is not a fix.

The hosted runner resolves its mirror through /etc/apt/apt-mirrors.txt, which
points at azure.archive.ubuntu.com. Point it at the canonical archive before
apt-get update, and retry update three times with backoff. A mirror fault now
costs seconds, not the whole budget, and a genuine outage fails loudly with a
clear message rather than a timeout.

The package list and install flags are unchanged.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: abrichr

.github/workflows/paper.yml

@@ -32,10 +32,35 @@ jobs:
         run: python paper/check_artifacts.py
       - name: Install TeX
         timeout-minutes: 10
-        run: >-
-          sudo apt-get update && sudo apt-get install -y
-          latexmk texlive-latex-extra texlive-pictures texlive-science
-          texlive-latex-recommended texlive-fonts-recommended
+        run: |
+          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 every failure costs minutes of
+          # apt retries before the canonical archive is tried. Prefer the
+          # canonical archive up front. Keep this best-effort: if the file is
+          # absent or already canonical, the run continues unchanged.
+          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
+          # A transient mirror fault must not fail the paper build outright.
+          update_ok=""
+          for attempt in 1 2 3; do
+            if sudo apt-get update; 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 \
+            latexmk texlive-latex-extra texlive-pictures texlive-science \
+            texlive-latex-recommended texlive-fonts-recommended
       - name: Build PDFs (full report + workshop condensation)
         timeout-minutes: 10
         run: make -C paper

public-artifacts.json

@@ -109,7 +109,7 @@
     },
     {
       "path": ".github/workflows/paper.yml",
-      "sha256": "4bd7904c18b1c29926f0fc6eff21a7401b8f3c41aa239622fba8f2dcbc362790"
+      "sha256": "6446edb96cddea116abe931611360122d8c4e3cba32e55c5ceab1f1e5209302a"
     },
     {
       "path": ".github/workflows/quickstart-lifecycle.yml",