[CI] (.github/actions, .github/workflows): run serial lanes at -j, retrace on the library alone

- unit, integration and integration-verify ctest run at -j nproc; the same labels already run at -j in the split lanes, with the same names and results
- build-linux, -split and -verify also publish the library (and the server image) alone; the retrace legs load nothing else, yet downloaded and unpacked the 0.5-0.8 GB test runtime
- build-retrace no longer waits for build-linux: it fetched the runtime only to test -f a path configure does not read
- every branch saves its own rolling ccache entry, deleting only its own old one (--ref); only dev saved, and dev has not run since the key rename, so every build was cold
- !cancelled() replaces job-level always() on the retrace, summary and skip-census jobs: a superseded run kept starting its pending legs and held the next run in the concurrency group for up to 154 minutes
- retrace and retrace-split run 8 legs at a time, not 4; the cap dates from per-leg LFS checkouts
- apt requests time out after 30 s and retry 3 times; install steps time out at 15 minutes, not 45
- the APK trace case matrix no longer waits for the build, which it does not read
- retrace-split prepares its metadata once, after the split library is unpacked

.github/actions/setup-test-runner/action.yml

@@ -38,4 +38,10 @@ runs:
             echo 'VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/lvp_icd.json'
             echo 'VK_DRIVER_FILES=/usr/share/vulkan/icd.d/lvp_icd.json'
           } >> "$GITHUB_ENV"
+          # A stalled mirror must fail a request and retry it, not hold the install step until
+          # its timeout.
+          if [ -d /etc/apt/apt.conf.d ] && command -v sudo > /dev/null; then
+            printf 'Acquire::Retries "3";\nAcquire::http::Timeout "30";\nAcquire::https::Timeout "30";\n' |
+              sudo tee /etc/apt/apt.conf.d/80-mobilegl-ci-network > /dev/null
+          fi
         fi