Add keyvault trigger when updating eng/common/testproxy (Azure#24774)

We auto-target `test-utils`, which is a good barometer, but I am only
comfortable with merging proxy changes if I can see that at least one
real library in each repo really works.

Author: scbedd

sdk/keyvault/ci.yml

@@ -25,6 +25,7 @@ pr:
   paths:
     include:
       - sdk/keyvault/
+      - eng/common/testproxy/
     exclude:
       - sdk/keyvault/ci.mgmt.yml
       - sdk/keyvault/arm-keyvault