fix: external sync actions not running

This bug was introduced by the refactor in commit 704940f,
which changed external_actions from a list to a key-value table.
However, when iterating through external_actions, we still use ipairs
which results in no values. The fix is to use pairs, which properly
iterates through all external_actions.

lua/rocks/operations/sync.lua

@@ -132,7 +132,7 @@ operations.sync = function(user_rocks, on_complete)
             end
 
             -- Sync actions handled by external modules that have registered handlers
-            for _, callback in ipairs(sync_status.external_actions) do
+            for _, callback in pairs(sync_status.external_actions) do
                 ct = ct + 1
                 callback(report_progress, report_error)
             end