test: change pip install command

Author: xNok

.github/workflows/solve_problem.yml

@@ -28,10 +28,9 @@ jobs:
           python -m pip install --upgrade pip
           if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
           if [ -f test-requirements.txt ]; then pip install -r test-requirements.txt; fi
-          python -m pip install ./models_or-tools
+          pip install ./models_or-tools
 
       - name: Run the script
         run: |
           pip freeze
-          cd ./models_or-tools/problems
-          python ${{ github.event.inputs.problem }}.py
+          python ./models_or-tools/problems/${{ github.event.inputs.problem }}.py