Only run eng/common check when target branch is master (#1415)

Only run eng/common check when target branch is master

eng/common/pipelines/templates/steps/eng-common-workflow-enforcer.yml

@@ -1,13 +1,24 @@
 steps:
   - ${{ if eq(variables['Build.Reason'], 'PullRequest') }}:
     - pwsh: |
-        $filesInCommonDir = git diff "origin/$(System.PullRequest.TargetBranch)" HEAD --name-only -- 'eng/common/*'
+        Set-PsDebug -Trace 1
+        # Find the default branch of the repo
+        $defaultBranch = (git remote show origin | Out-String) -replace "(?ms).*HEAD branch: (\w+).*", '$1'
+        $targetBranch = "$(System.PullRequest.TargetBranch)" -replace "refs/heads/"
 
-        if ((!"$(System.PullRequest.SourceBranch)".StartsWith("sync-eng/common")) -and ($LASTEXITCODE -eq 0) -and ($filesInCommonDir.Count -gt 0))
+        Write-Host "Default Branch: ${defaultBranch}"
+
+        if ((!"$(System.PullRequest.SourceBranch)".StartsWith("sync-eng/common")) -and $targetBranch -eq $defaultBranch)
+        {
+
+          $filesInCommonDir = git diff "origin/${targetBranch}" HEAD --name-only -- 'eng/common/*'
+
+          if (($LASTEXITCODE -eq 0) -and ($filesInCommonDir.Count -gt 0))
           {
             Write-Host "##vso[task.LogIssue type=error;]Changes to files under 'eng/common' directory should not be made in this Repo`n${filesInCommonDir}"
             Write-Host "##vso[task.LogIssue type=error;]Please follow workflow at https://github.com/Azure/azure-sdk-tools/blob/master/doc/common/common_engsys.md"
             exit 1
           }
+        }
       displayName: Prevent changes to eng/common outside of azure-sdk-tools repo
       condition: and(succeeded(), ne(variables['Skip.EngCommonWorkflowEnforcer'], 'true'))