spindle/engines/nixery: fix panic when parsing environment variables

Signed-off-by: Evan Jarrett <[e-mail]>

spindle/engines/nixery/engine.go

···
 		workflowEnvs.AddEnv(s.Key, s.Value)
 	}
 
-	step := w.Steps[idx].(Step)
+	step := w.Steps[idx]
 
 	select {
 	case <-ctx.Done():
···
 	}
 
 	envs := append(EnvVars(nil), workflowEnvs...)
-	for k, v := range step.environment {
-		envs.AddEnv(k, v)
+	if nixStep, ok := step.(Step); ok {
+		for k, v := range nixStep.environment {
+			envs.AddEnv(k, v)
+		}
 	}
 	envs.AddEnv("HOME", homeDir)