Support named pipes when loading .env files (#70725)

### What?

Updates `@next/env` to support loading `.env` "files" that are named
pipes.

### Why?

Some third party secret managers (like
[Doppler](https://docs.doppler.com/docs/accessing-secrets#mounting))
have CLIs that can "mount" a `.env` file. This "file" is actually a
named pipe, which prevents the secrets from being stored on disk, but
otherwise acts exactly like a file. The check in `@next/env` prevents
this from working because `isFile()` returns false for named pipes.

### How?

When loading potential `.env` files, an `fs.statSync()` is performed and
then `isFile()` is called. If that's false, the potential file is
skipped. This includes named pipes. The change is very simple and just
updates the check so it looks at `isFile()` _and_ `isFIFO()`, which
returns true for named pipes.

Co-authored-by: JJ Kasper <[email protected]>

Author: watsonian

packages/next-env/index.ts

@@ -148,8 +148,8 @@ export function loadEnvConfig(
     try {
       const stats = fs.statSync(dotEnvPath)
 
-      // make sure to only attempt to read files
-      if (!stats.isFile()) {
+      // make sure to only attempt to read files or named pipes
+      if (!stats.isFile() && !stats.isFIFO()) {
         continue
       }