fix: bring back support for build-time env vars

[JakeChampion]

We had these working before we introduced the two experimental flags, when we added those that meant the default env inheritance was overridden and we never added it back

here is it working:

❯ command cat src/index.js
/// <reference types="@fastly/js-compute" />

import { env } from "fastly:env";

const pwd = env("DOG")

addEventListener("fetch", (event) => event.respondWith(handleRequest(event)));

async function handleRequest(event) {
  console.log({buildtime: pwd}, {runtime:env("DOG")});
  return new Response("OK", { status: 200 });
}
❯ export DOG=woof
❯ fastly compute serve --verbose
Fastly API token provided via config file (profile: work)
Fastly API endpoint: https://api.fastly.com

✓ Verifying fastly.toml
✓ Identifying package name
✓ Identifying toolchain

INFO: Creating ./bin directory (for Wasm binary)

Build script to execute:
        sh -c npm run build


INFO: Command output:
--------------------------------------------------------------------------------

> build
> js-compute-runtime ./src/index.js ./bin/main.wasm

--------------------------------------------------------------------------------

✓ Running [scripts.build]

Wasm module 'magic':
        []byte{0x0, 0x61, 0x73, 0x6d}

Wasm module 'version':
        1

✓ Creating package archive

SUCCESS: Built package (pkg/env-meow.tar.gz)


INFO: Viceroy is installed but the CLI config (`fastly config`) shows the TTL, checking for a newer version,
hasn't expired. To force a refresh, re-run the command with the `--viceroy-check` flag.

✓ Running local server

Manifest: /Users/jakechampion/temp/dora-env/fastly.toml
Wasm binary: bin/main.wasm
Viceroy binary: /Users/jakechampion/Library/Application Support/fastly/viceroy
Viceroy version: viceroy 0.9.1

INFO: Command output:
--------------------------------------------------------------------------------
2023-10-16T13:08:41.773188Z  WARN no backend definitions found in /Users/jakechampion/temp/dora-env/fastly.toml
2023-10-16T13:08:41.773307Z  INFO Listening on http://127.0.0.1:7676
2023-10-16T13:08:43.484188Z  INFO request{id=0}: handling request GET http://localhost:7676/
Log: { buildtime: "woof" } { runtime: "" }
2023-10-16T13:08:43.486914Z  INFO request{id=0}: request completed using 7.7 MB of WebAssembly heap
2023-10-16T13:08:43.486921Z  INFO request{id=0}: request completed in 3ms

[doramatadora]

Thank you ❤️