Use the right branch for doctest testing

Author: rocky

.github/workflows/mathics.yml

@@ -30,6 +30,8 @@ jobs:
         make
     - name: Test Mathics3
       run: |
+        git clone --depth 1 --branch revise-escape-sequence-scanning https://github.com/Mathics3/mathics-scanner.git
+        (cd mathics-scanner && pip install -e .)
         # Until next Mathics3/mathics-core release is out...
         git clone https://github.com/Mathics3/mathics-core.git
         cd mathics-core/