How to set env for nextjs ?

Hi,

I have a nextJS app and using the nextjs macros (thank you so much for adding these btw! )

```
nextjs(
name = "next",
srcs = [
"//src/app",
],
config = "next.config.mjs",
data = [
"//:node_modules/next",
"//:node_modules/react",
"//:node_modules/react-dom",
"//:node_modules/styled-jsx",
"//:node_modules/typescript",
"//:package_json",
],
next_js_binary = "//:next_js_binary",
)
```

This works perfectly fine when I do `bazel run next.dev`. However, if I do
`bazel run next.start`, I have issues when nextJS tries to pre-render the page (e.g. `/`). Specifically, some required environment variables are not set.

How do I make sure that the env variables are set (i.e. available in `process.env`) when nextJS is pre-rendering some routes during `next.start`?

____

I tried playing around with passing `env = {'MY_VAR': 'xxx'}` in the `nextjs` rule, and that works, but 'xxx' must be hardcoded, so that I cannot really use it (`MY_VAR` can be some API token, for example, so I cannot hardcode it into the `BUILD` file).

I tried also using `bazel run --action_env=MY_VAR next.start` but here it's also not able to find the `MY_VAR` definition.

What is the recommended approach here?

Thanks a lot for your help!

Start with the nextjs rule invocation and the next.start execution path described in the issue. Reproduce the environment-variable behavior, comparing the env attribute with bazel run --action_env=MY_VAR. Done means identifying and documenting or implementing a supported way to make runtime variables available during Next.js prerendering without hardcoding secrets in BUILD.

Written by the indexing model from the issue text.