Update an existing array element with jsonnet

I am using jsonnet to read in a value which consists of an array. I want to modify the first element in that array to add a value. The data structure looks like this:

{
   "my_value": [
      {
         "env": "something"
      },
      {
         "var": "bar"
      }
   ]
}

I want to add a value to my_value[0]. How can I reference that in jsonnet?

A possible approach using https://jsonnet.org/ref/stdlib.html#mapWithIndex as per below:

$ cat foo.jsonnet 
local my_array = [
  {
    env: "something",
  },
  {
    var: "bar",
  },
];
local add_by_idx(idx) = (
  if idx == 0 then { extra: "stuff" } else {}
);
std.mapWithIndex(function(i, v) v + add_by_idx(i), my_array)

$ jsonnet foo.jsonnet 
[
   {
      "env": "something",
      "extra": "stuff"
   },
   {
      "var": "bar"
   }
]