fix: updated the workflow to checkout target branch specified by the user (#26705)

.github/workflows/upgrade-python-requirements.yml

@@ -20,7 +20,13 @@ jobs:
         python-version: ["3.8"]
 
     steps:
+      - name: setup target branch
+        run: echo "target_branch=$(if ['${{ github.event.inputs.branch }}' = '']; then echo 'master'; else echo '${{ github.event.inputs.branch }}'; fi)" >> $GITHUB_ENV
+
       - uses: actions/checkout@v1
+        with:
+          ref: ${{ env.target_branch }}
+
       - name: setup python
         uses: actions/setup-python@v2
         with:
@@ -34,9 +40,6 @@ jobs:
           cd $GITHUB_WORKSPACE
           make upgrade
 
-      - name: setup target branch
-        run: echo "target_branch=$(if ['${{ github.event.inputs.branch }}' = '']; then echo 'master'; else echo '${{ github.event.inputs.branch }}'; fi)" >> $GITHUB_ENV
-
       - name: setup testeng-ci
         run: |
           git clone https://github.com/edx/testeng-ci.git