fix(cli): set `npm_config_user_agent` when running npm packages or tasks (#26639)

Fixes #25342.

Still not sure on the exact user agent to set (should it include
`node`?).

After this PR, here's the state of running some `create-*` packages
(just ones I could think of off the top of my head):

| package                  | prints/runs/suggests deno install | notes |
| ---------------- | ------------- | ------ |
| `create-next-app` | ❌ | falls back to npm, needs a PR
([code](https://github.com/vercel/next.js/blob/c32e2802097c03fd9f95b1dae228d6e0257569c0/packages/create-next-app/helpers/get-pkg-manager.ts#L3))
| `sv create` | ❌ | uses `package-manager-detector`, needs a PR
([code](https://github.com/antfu-collective/package-manager-detector/tree/main))
| `create-qwik` | ✅ | runs `deno install` but suggests `deno start`
which doesn't work (should be `deno task start` or `deno run start`)
| `create-astro` | ✅ | runs `deno install` but suggests `npm run dev`
later in output, probably needs a PR
| `nuxi init` | ❌ | deno not an option in dialog, needs a PR
([code](https://github.com/nuxt/cli/blob/f04e2e894446f597da9d971b7eb03191d5a0da7e/src/commands/init.ts#L96-L102))
| `create-react-app` | ❌ | uses npm
| `ng new` (`@angular/cli`) | ❌ | uses npm
| `create-vite` | ✅ | suggests working deno commands 🎉 
| `create-solid` |  ❌ | suggests npm commands, needs PR

It's possible that fixing `package-manager-detector` or other packages
might make some of these just work, but haven't looked too carefully at
each

1 files changed, 1 insertions, 0 deletions

diff --git a/tests/specs/npm/user_agent_env_var/test.mjs b/tests/specs/npm/user_agent_env_var/test.mjs
new file mode 100644
index 000000000..ae035d40e
--- /dev/null
+++ b/tests/specs/npm/user_agent_env_var/test.mjs
@@ -0,0 +1 @@
+console.log(process.env.npm_config_user_agent);