[CI] Undo a merge done by Github/VSTS when we do the checkout so that both jobs (build & test) use the same commit. (#14443)

tools/devops/automation/templates/build/build.yml

@@ -49,6 +49,13 @@ steps:
   submodules: recursive
   path: s/xamarin-macios
 
+
+- ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
+  - pwsh: |
+      git reset --hard HEAD^2 
+    displayName: "Undo Github merge"
+    workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-macios
+
 - checkout: maccore
   clean: true
   persistCredentials: true  # hugely important, else there are some scripts that check a single file from maccore that will fail

tools/devops/automation/templates/tests/build.yml

@@ -51,6 +51,13 @@ steps:
 
 - checkout: self
   path: s/xamarin-macios
+
+- ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
+  - pwsh: |
+      git reset --hard HEAD^2 
+    displayName: "Undo Github merge"
+    workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-macios
+
 - checkout: maccore
   persistCredentials: true  # hugely important, else there are some scripts that check a single file from maccore that will fail