Add SANDSTORM=1 to the environment under Sandstorm

We add the SANDSTORM=1 environment variable to sandstorm-pkgdef.capnp so that
accounts-sandstorm will ensure that it's the only loaded accounts package when
running under Sandstorm.

sandstorm-pkgdef.capnp

@@ -184,6 +184,7 @@ const myCommand :Spk.Manifest.Command = (
   environ = [
     # Note that this defines the *entire* environment seen by your app.
     (key = "PATH", value = "/usr/local/bin:/usr/bin:/bin"),
+    (key = "SANDSTORM", value = "1"),
     (key = "METEOR_SETTINGS", value = "{\"public\": {\"sandstorm\": true}}")
   ]
 );