Do not use env key in Babel common preset (#48661)

The merging behavior of having both `plugins` and `env.test.plugins` in the Babel configuration is confusing and does not work how we would like (that is, disabling the idx Babel plugin in test environments.)

There had been some [discussion about deprecating the `env` key in Babel](https://github.com/babel/babel/issues/5276) because of this behavior and recommending using conditional logic. The behavior has been modified in later versions of Babel, but still does not work as intended in our case.

Change this to only add the idx plugin if we're in a non-test environment.

packages/kbn-babel-preset/common_preset.js

@@ -28,17 +28,17 @@ const plugins = [
   // See https://github.com/babel/proposals/issues/12 for progress
   require.resolve('@babel/plugin-proposal-class-properties'),
 ];
+const isTestEnv = process.env.BABEL_ENV === 'test' || process.env.NODE_ENV === 'test';
+
+// Only load the idx plugin in non-test environments, since it conflicts with
+// Jest's coverage mapping.
+if (!isTestEnv) {
+  plugins.push(require.resolve('@kbn/elastic-idx/babel'));
+}
 
 module.exports = {
   presets: [require.resolve('@babel/preset-typescript'), require.resolve('@babel/preset-react')],
-  plugins: plugins.concat(require.resolve('@kbn/elastic-idx/babel')),
-  // Do not use the idx plugin in the test environment because it causes
-  // causes conflicts with Jest's coverage mapping.
-  env: {
-    test: {
-      plugins,
-    },
-  },
+  plugins,
   overrides: [
     {
       // Babel 7 don't support the namespace feature on typescript code.