[CI] Specify our PR build trigger in YAML. (#598)

It appears the one specified in the AzDO UI is no longer functioning.

Author: jpobst

build-tools/automation/azure-pipelines.yaml

@@ -4,6 +4,10 @@ trigger:
   - master
   - d16-*
 
+pr:
+  - master
+  - d16-*
+
 # Global variables
 variables:
   DotNetCoreVersion: 3.1.100