feat: add environment variable for esbuild (#1914)

Author: eduardoboucas

src/lib/build.js

@@ -1,3 +1,5 @@
+const { env } = require('process')
+
 const build = require('@netlify/build')
 
 // We have already resolved the configuration using `@netlify/config`
@@ -10,7 +12,19 @@ const getBuildOptions = ({ context, token, flags }) => {
   // buffer = true will not stream output
   const buffer = flags.json || flags.silent
 
-  return { cachedConfig: serializedConfig, token, dry, debug, mode: 'cli', telemetry: false, buffer }
+  // @todo Remove once esbuild has been fully rolled out.
+  const featureFlags = env.NETLIFY_EXPERIMENTAL_ESBUILD ? 'buildbot_esbuild' : undefined
+
+  return {
+    cachedConfig: serializedConfig,
+    token,
+    dry,
+    debug,
+    mode: 'cli',
+    telemetry: false,
+    buffer,
+    featureFlags,
+  }
 }
 
 const runBuild = async (options) => {