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':

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;
       }