/home/runner/work/CST-Python/CST-Python/./.github/workflows/test_inner/action.yml (Line: 21, Col: 7): Required property is missing: shell

GitHub.DistributedTask.ObjectTemplating.TemplateValidationException: The template is not valid. /home/runner/work/CST-Python/CST-Python/./.github/workflows/test_inner/action.yml (Line: 21, Col: 7): Required property is missing: shell at GitHub.DistributedTask.ObjectTemplating.TemplateValidationErrors.Check() at GitHub.Runner.Worker.ActionManifestManager.ConvertRuns(IExecutionContext executionContext, TemplateContext templateContext, TemplateToken inputsToken, String fileRelativePath, MappingToken outputs) at GitHub.Runner.Worker.ActionManifestManager.Load(IExecutionContext executionContext, String manifestFile)