Fix bashisms

Author: dalcinl

.github/workflows/ci.yml

@@ -176,26 +176,23 @@ jobs:
       uses: ./
       with:
         mpi: mpich
-    - run: test ${{ steps.setup1.outputs.mpi }} == mpich
-      shell: bash
+    - run: test ${{ steps.setup1.outputs.mpi }} = mpich
     - run: command -v mpichversion
 
     - name: Setup Open MPI
       id:   setup2
       uses: ./
       with:
         mpi: openmpi
-    - run: test ${{ steps.setup2.outputs.mpi }} == openmpi
-      shell: bash
+    - run: test ${{ steps.setup2.outputs.mpi }} = openmpi
     - run: command -v ompi_info
 
     - name: Setup Intel MPI
       id:   setup3
       uses: ./
       with:
         mpi: intelmpi
-    - run: test ${{ steps.setup3.outputs.mpi }} == intelmpi
-      shell: bash
+    - run: test ${{ steps.setup3.outputs.mpi }} = intelmpi
     - run: command -v impi_info
 
   macOS: