fix shell selection

Author: wpbonelli

action.yml

@@ -45,10 +45,7 @@ runs:
     - uses: mamba-org/setup-micromamba@v1
       if: runner.os == 'macOS' && contains(inputs.compiler, 'lfortran')
       with:
-        init-shell: >-
-          bash
-          powershell
-          cmd.exe
+        init-shell: bash
         post-cleanup: 'all'
 
     # Set up the selected toolchain or compiler