Merged PR 741263: Increase fetch depth for public PR validation to get cachemiss analysis

bxl.ps1 adds /cachemiss for the last 3 commits. But this job is configured to just fetch the last commit, meaning /cachemiss never has anything to act upon since it's just looking up its own commit

.azdo/pr/public_pr.yml

@@ -32,7 +32,7 @@ jobs:
     name: BuildXL-DevOpsAgents-PME
   steps:
   - checkout: self
-    fetchDepth: 1
+    fetchDepth: 3
 
   - template: ../common/set-artifacts-pat.yml
     parameters: