make valgrind run on every pr

.ci-builds/.azure-valgrind.yml

diff --git a/.ci-builds/.azure-valgrind.yml b/.ci-builds/.azure-valgrind.yml
index 63d8718..ced5892 100644
--- a/.ci-builds/.azure-valgrind.yml
+++ b/.ci-builds/.azure-valgrind.yml
@@ -9,8 +9,7 @@
 # Only run valgrind on the main and maint branches, and
 # batch up any pending changes so that we will only have at most
 # one build running at a given time (since it takes time).
-trigger:
-  batch: true
+pr:
   branches:
     include:
     - main