Set CI cache download timeout to enable retries

Default timeout is 60 minutes, after which a stuck download would be
retried. Lower it to start retrying downloads when they get stuck.

.github/workflows/ci.yml

@@ -31,6 +31,8 @@ env:
   # This value should be greater than the time taken for the longest image pull.
   TESTCONTAINERS_PULL_PAUSE_TIMEOUT: 600
   TEST_REPORT_RETENTION_DAYS: 5
+  # used by actions/cache to retry the download after this time: https://github.com/actions/cache/blob/main/workarounds.md#cache-segment-restore-timeout
+  SEGMENT_DOWNLOAD_TIMEOUT_MINS: 5
 
 # Cancel previous PR builds.
 concurrency: