[release/6.0] Fix branch names again (#42142)

* Fix branch names again

* Update azure-pipelines-mirror-within-azdo.yml

* Update azure-pipelines-mirror-within-azdo.yml

Co-authored-by: William Godbe <wigodbe@microsoft.com>

.azure/pipelines/azure-pipelines-mirror-within-azdo.yml

@@ -33,11 +33,13 @@ jobs:
           value: repo-dir
         - name: AzdoRepo
           value: dotnet-aspnetcore
-        - name: TargetBranchName
-          value: $(Build.SourceBranchName)-nonstable
-        - name: BranchToMirror
-          value: $(Build.SourceBranchName)
         steps:
+        - powershell: |
+            $branch = "$(Build.SourceBranch)".Replace("refs/heads/", "");
+            $suffix = "-nonstable"
+            Write-Host "##vso[task.setvariable variable=BranchToMirror]$branch"
+            Write-Host "##vso[task.setvariable variable=TargetBranchName]$branch+$suffix"
+          displayName: Calculate Mirrored Branch Names
         - script: |
             git clone https://dn-bot:$(dn-bot-dnceng-build-rw-code-rw)@dev.azure.com/dnceng/internal/_git/$(AzdoRepo) $(WorkingDirectoryName) --recursive --no-tags --branch $(TargetBranchName)
           displayName: Clone AzDO repo