Fix parsing of environment variables when merging env into config

CHANGELOG.md

@@ -4,6 +4,10 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 ## Master
 
+- Environment variables now have to **start** with `YARN_` (instead of just contain it) to be considered
+
+  [#6518](https://github.com/yarnpkg/yarn/pull/6518) - [**Michael Gmelin**](https://blog.grem.de)
+
 - Fixes the `extensions` option when used by `resolveRequest`
 
   [#6479](https://github.com/yarnpkg/yarn/pull/6479) - [**Maël Nison**](https://twitter.com/arcanis)

src/registries/base-registry.js

@@ -151,7 +151,7 @@ export default class BaseRegistry {
       let key = envKey.toLowerCase();
 
       // only accept keys prefixed with the prefix
-      if (key.indexOf(prefix.toLowerCase()) < 0) {
+      if (key.indexOf(prefix.toLowerCase()) !== 0) {
         continue;
       }