[llvm] [CI] Make premerge_advisor_explain write comments (PR #166605)

[David Spickett via llvm-commits]

================
@@ -33,17 +33,18 @@ function at-exit {
   # If building fails there will be no results files.
   shopt -s nullglob
 
-  if [[ "$GITHUB_STEP_SUMMARY" != "" ]]; then
+  if [[ "$GITHUB_ACTIONS" != "" ]]; then
----------------
DavidSpickett wrote:

I see, so finding GITHUB_STEP_SUMMARY is basically a side effect of what we actually want to check for. So the new check is clearer to anyone who sees it for the first time :+1: 

https://github.com/llvm/llvm-project/pull/166605