[cron] Fix typo redir erasing logfile

Change-Id: I1c929d4119145830044468d1752b4aa7a6580bea
diff --git a/helpers/llvm-cron-build b/helpers/llvm-cron-build
index d433903..4751946 100755
--- a/helpers/llvm-cron-build
+++ b/helpers/llvm-cron-build
@@ -80,4 +80,4 @@
 # Call llvm-build (no tests wanted)
 env CC="$CC" CXX="$CXX" CFLAGS="$CFLAGS" CXXFLAGS="$CXXFLAGS" llvm-build >> "$log" 2>&1
 
-echo "Finish date: $(date)" > "$log"
+echo "Finish date: $(date)" >> "$log"