ICU-21244 Make Valgrind CI build/test check run on every Pull Request

See #1713

.ci-builds/.azure-valgrind.yml

diff --git a/.ci-builds/.azure-valgrind.yml b/.ci-builds/.azure-valgrind.yml
index 63d8718..e7abc8c 100644
--- a/.ci-builds/.azure-valgrind.yml
+++ b/.ci-builds/.azure-valgrind.yml
@@ -1,16 +1,14 @@
 # Azure Pipelines configuration for Valgrind for ICU4C.
 # 
 # Note: The valgrind test configuration is in a separate file
-# so that it can be run independently from the regular builds.
+# as it used to be run independently from the regular CI builds.
 #
 # The Ubuntu images don't have valgrind installed by default, so we need
 # install it first.
 #
-# 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
+# Run Valgrind for every Pull Request that goes into main
+# or a maint- branch. This lets us catch any issues sooner rather than later. 
+pr:
   branches:
     include:
     - main