clean up build dir and pick correct working dir when building pymarian.

Author: erip

.github/workflows/macos.yml

@@ -50,14 +50,15 @@ jobs:
         ./marian-decoder --version
         ./marian-scorer --version
         ls -hlv $(find . -maxdepth 1 -type f -perm +ugo+x \( -name "marian*" -o -name "spm*" \))
+        cd ..
+        rm -rf build/
 
     - name: Install PyMarian
-      working-directory: build
+      working-directory: src/python
       run: |
         python3 -m venv .venv && source .venv/bin/activate
         python3 -m pip install -U setuptools wheel
         python3 -m pip install pybind11 sentencepiece scikit-build
-        cd ../src/python
         python3 setup.py install -j2
         cd ../../
         python3 -c "from pymarian import Translator"