CI: stop recurring false failures on main and pull requests

- checks: the runner's packages.microsoft.com apt source now 403s, failing
  `apt-get update` and with it "Install zsh"; tolerate the refresh error
  (Ubuntu's lists still update and the install still fails if it can't).
- e2e: a failed quiet image build is rebuilt with the full log, but the
  script exited 2 even when that rebuild succeeded; only fail if it fails.
- ArtworkTests: a timed-out lookup gives its resolver slot back from its own
  thread, which a busy macOS runner may schedule after the next test starts.
  Wait for every slot before each test, and always return slots a test took,
  so one late thread can't cascade into three failures.
- release: tagged builds no longer re-upload the installers as workflow
  artifacts (they are attached to the release; the artifact finalize timed
  out on v0.3.0). Pull requests build installers only when the Electron app,
  the macOS helper it compiles, or the workflow changes.
- checks and release: a newer push to a pull request cancels the older run.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/checks.yml

@@ -5,6 +5,11 @@ on:
     branches: [main]
   pull_request:
 
+# A newer push to the same pull request supersedes a run still in progress.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   checks:
     runs-on: ubuntu-latest
@@ -17,7 +22,9 @@ jobs:
         with:
           node-version: "24"
       - name: Install zsh
-        run: sudo apt-get update -qq && sudo apt-get install -y -qq zsh
+        # The runner image's third-party apt sources (packages.microsoft.com) sometimes
+        # fail to refresh; Ubuntu's own lists still update, and the install fails if not.
+        run: (sudo apt-get update -qq || true) && sudo apt-get install -y -qq zsh
       - name: Script syntax
         run: |
           sh -n ui/local-bin/ssh
@@ -84,6 +91,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Install zsh
-        run: sudo apt-get update -qq && sudo apt-get install -y -qq zsh
+        # The runner image's third-party apt sources (packages.microsoft.com) sometimes
+        # fail to refresh; Ubuntu's own lists still update, and the install fails if not.
+        run: (sudo apt-get update -qq || true) && sudo apt-get install -y -qq zsh
       - name: End-to-end tests
         run: scripts/e2e.sh