Enable in-build signing based on branch (#81498)

Co-authored-by: Matt Mitchell <[email protected]>

Author: github-actions[bot]

eng/pipelines/common/internal-variables.yml

@@ -4,5 +4,9 @@ parameters:
 variables:
   - name: TeamName
     value: ${{ parameters.teamName }}
-  - name: PostBuildSign
-    value: true
+  - ${{ if or(startswith(variables['Build.SourceBranch'], 'refs/heads/release/'), startswith(variables['Build.SourceBranch'], 'refs/heads/internal/release/'), eq(variables['Build.Reason'], 'Manual')) }}:
+    - name: PostBuildSign
+      value: false
+  - ${{ else }}:
+    - name: PostBuildSign
+      value: true