[ci] add missing shell property, this seems to be necessary when usin…

…g composite actions

.github/actions/run_tests/action.yml

@@ -20,4 +20,5 @@ runs:
         name: psyc_${{ inputs.compiler }}_${{ inputs.build_config }}
 
     - name: scratchpad.psy
+      shell: bash
       run: psyc -v ./samples/scratchpad.psy --dump-ast --dump-ir