Fix github-token-refresh test: remove illegal double snapshot call

The test was calling snapshot twice within a single task, which is
illegal (snapshot writes to a single IORef that can only hold one
value). Fixed by removing the second snapshot call and relying on
the automatic final status update when the task completes.

Test flow now:
1. snapshot posts "pending" status (creates and caches token)
2. Task sleeps 3 seconds (token expires after 2 seconds)
3. Task completes, final "success" status is posted automatically
   (detects expired cached token and refreshes it)

This properly tests token refresh while following the one-snapshot-per-task rule.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: zyla

test/t/slow/github-token-refresh.out

@@ -1,12 +1,8 @@
 -- output:
-[mytask] stdout | First status update done
-[mytask] stdout | Second status update done (token refreshed)
+[mytask] stdout | Task started, pending status posted
+[mytask] stdout | Task finishing (token will be refreshed for final status)
 -- github:
 Requested access token for installation 123
-Requested access token for installation 123
-Updated commit status for fakeowner/fakerepo to {"context":"mytask","description":"not cached","state":"pending","target_url":null}
-Requested access token for installation 123
 Updated commit status for fakeowner/fakerepo to {"context":"mytask","description":"not cached","state":"pending","target_url":null}
 Requested access token for installation 123
-Requested access token for installation 123
 Updated commit status for fakeowner/fakerepo to {"context":"mytask","description":null,"state":"success","target_url":null}

test/t/slow/github-token-refresh.txt

@@ -10,8 +10,7 @@ git commit --allow-empty -q -m "Initial commit"
 
 taskrunner -n mytask bash -e -c '
   snapshot -n --commit-status
-  echo "First status update done"
+  echo "Task started, pending status posted"
   sleep 3
-  snapshot -n --commit-status
-  echo "Second status update done (token refreshed)"
+  echo "Task finishing (token will be refreshed for final status)"
 '