_shim leaks into exec error messages

[shykes]

When executing a command in a container, cloak inserts an entrypoint called _shim. This fact should be hidden from users, but it leaks through in error messages. For example this query:

{
 core {
  image(ref: "alpine") {
   exec(input: {
    args: ["/bin/ls", "does-not-exist"]
   }) {
    id
   }
  }
 }
}

Returns this result:

{
  "data": null,
  "errors": [
    {
      "message": "process \"/_shim /bin/ls does-not-exist\" did not complete successfully: exit code: 1",
      "locations": [
        {
          "line": 4,
          "column": 7
        }
      ],
      "path": [
        "core",
        "image",
        "exec"
      ]
    }
  ]
}