[release/8.0.1xx] Check out the correct target VMR branch (#17656)

eng/pipelines/templates/jobs/vmr-build.yml

@@ -90,7 +90,10 @@ jobs:
   steps:
   - template: ../steps/vmr-prepare.yml
     parameters:
-      vmrBranch: ${{ parameters.vmrBranch }}
+      ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
+        vmrBranch: $(System.PullRequest.TargetBranch)
+      ${{ else }}:
+        vmrBranch: ${{ parameters.vmrBranch }}
       isBuiltFromVmr: ${{ parameters.isBuiltFromVmr }}
       skipComponentGovernanceDetection: true
 

eng/pipelines/templates/steps/vmr-prepare.yml

@@ -28,11 +28,13 @@ steps:
     path: vmr
     clean: true
 
-- ${{ if or(not(parameters.isBuiltFromVmr), eq(variables['System.TeamProject'], 'internal')) }}:
-  - script: |
-      git switch -c ${{ parameters.vmrBranch }}
-    displayName: Checkout ${{ parameters.vmrBranch }}
-    workingDirectory: $(Agent.BuildDirectory)/vmr
+- script: |
+    set -euxo pipefail
+    git fetch --all
+    git checkout --track origin/${{ parameters.vmrBranch }}
+    echo "##vso[task.setvariable variable=vmrBranch]${{ parameters.vmrBranch }}"
+  displayName: Check out ${{ parameters.vmrBranch }}
+  workingDirectory: $(Agent.BuildDirectory)/vmr
 
 # TODO (https://github.com/dotnet/arcade/issues/11332): Allow full CG?
 # Currently, we ignore dirs of individual repos - they have been scanned before

eng/pipelines/templates/steps/vmr-pull-updates.yml

@@ -22,12 +22,6 @@ steps:
   displayName: Clone dotnet/installer
   path: installer
 
-- script: |
-    git checkout -B ${{ parameters.vmrBranch }}
-    echo "##vso[task.setvariable variable=vmrBranch]${{ parameters.vmrBranch }}"
-  displayName: Prepare branch ${{ parameters.vmrBranch }}
-  workingDirectory: ${{ parameters.vmrPath }}
-
 - script: >
     ./eng/vmr-sync.sh
     --vmr ${{ parameters.vmrPath }}