Only update the cache in one job on mainnet

.github/workflows/continous-integration-docker.yml

@@ -441,8 +441,8 @@ jobs:
       test_variables: '-e GENERATE_CHECKPOINTS_MAINNET=1 -e ZEBRA_FORCE_USE_COLOR=1 -e ZEBRA_CACHED_STATE_DIR=/var/cache/zebrad-cache'
       network: 'Mainnet'
       needs_zebra_state: true
-      # update the disk on every PR, to increase CI speed
-      saves_to_disk: true
+      # test-update-sync updates the disk on every PR, so we don't need to do it here
+      saves_to_disk: false
       disk_suffix: tip
       root_state_path: '/var/cache'
       zebra_state_dir: 'zebrad-cache'
@@ -525,6 +525,7 @@ jobs:
       network: 'Testnet'
       needs_zebra_state: true
       # update the disk on every PR, to increase CI speed
+      # we don't have a test-update-sync-testnet job, so we need to update the disk here
       saves_to_disk: true
       disk_suffix: tip
       root_state_path: '/var/cache'