Update test_mpc.yml (#149)

Author: jorgensd

.github/workflows/test_mpc.yml

@@ -97,7 +97,7 @@ jobs:
           petsc_arch: ${{ env.PETSC_ARCH }}
       - name: Restrict CFFI
         run:
-           python3 -m pip install --no-build-isolation cffi<1.17
+           python3 -m pip install --no-build-isolation "cffi<1.17"
 
       - name: Install DOLFINx-MPC (C++)
         run: |