Allow to use the environment variable STEP_CA_TOKEN

For helm charts we want to store the tokens in a secret and load
it from an environment variable.

commands/app.go

@@ -41,6 +41,7 @@ certificate issuer private key used in the RA mode.`,
 		cli.StringFlag{
 			Name:   "token",
 			Usage:  "token used to enable the linked ca.",
+			EnvVar: "STEP_CA_TOKEN",
 		},
 	},
 }