Wolfram CI retries a stuck package mirror

On several runs, the Install system dependencies step hung on a package mirror. Nothing failed; the jobs simply sat for an hour. The release workflow waits on CI gate, so v0.37.0 was blocked until a run was cancelled and re-run.

Change

Each apt-get update runs under a 300 s timeout and each apt-get install under 900 s. The pair is retried up to three times, with a short pause between attempts. If all three fail, the step fails rather than waiting. This is #186.

The retry cannot help a run that is already stuck: it uses the workflow file from that run's commit. The re-run is what unblocks it.