Fix transformer changing process.env.BABEL_ENV as a side-effect, transform tests non-determinism

Summary:
X-link: facebook/react-native#55557

Currently, the RN Babel transformer that is used both by Metro and Jest, and the base transformer in Metro, set [`process.env.BABEL_ENV`](https://babeljs.io/docs/options?utm_source=chatgpt.com#envname) during transformation, presumably to ensure that Babel always uses the `development` configuration when multiple are configured.

It attempts to set it back in a `finally` block - but that's guarded in the old value being truthy, which was introduced as a fix in #446 - setting `process.env.BABEL_ENV = undefined` actually gives it the string value `'undefined'`.

However, the fix wasn't quite right, because it just leaves a previously-unset `BABEL_ENV` at its new value. So if `BABEL_ENV` is initially `undefined`, using the transformer sets it to `'development'`.

This has a knock-on effect on `react-native/babel-preset`, which sets `dev: true` on the preset if `BABEL_ENV` is `'development'`.

Changelog:
[General][Fixed] Fix Babel transformer changing `BABEL_ENV` as a side effect

Reviewed By: huntie

Differential Revision: D93328555

fbshipit-source-id: e47f51028e60efc1c346bea44d1d8f2af264ca60

Author: robhogan

packages/metro-babel-transformer/src/index.js

@@ -130,7 +130,14 @@ function transform({
       metadata: transformResult.metadata,
     };
   } finally {
-    if (OLD_BABEL_ENV) {
+    // Restore the old process.env.BABEL_ENV
+    if (OLD_BABEL_ENV == null) {
+      // We have to treat this as a special case because writing undefined to
+      // an environment variable coerces it to the string 'undefined'. To
+      // unset it, we must delete it.
+      // See https://github.com/facebook/metro/pull/446
+      delete process.env.BABEL_ENV;
+    } else {
       process.env.BABEL_ENV = OLD_BABEL_ENV;
     }
   }