Environment variable prefixes should start at the beginning, not in t…

…he middle of an environment variable. This fixes this example:

  SOME_YARN_CONFIG=123 yarn config list | grep some
    'some-yarn-config':
  { 'some-yarn-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
 
+- Fix parsing of environment variables when merging env into config
+
+  [#6518](https://github.com/yarnpkg/yarn/pull/6518) - [**Michael Gmelin**](https://blog.grem.de)
+
 - Adds support for basic auth for registries with paths, such as artifactory
 
   [#5322](https://github.com/yarnpkg/yarn/pull/5322) - [**Karolis Narkevicius**](https://twitter.com/KidkArolis)

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;
       }