fix: drive letter casing for win32 pnp (#7007)

* fix: drive letter casing for win32 pnp

* docs: changelog

CHANGELOG.md

@@ -4,6 +4,10 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 ## Master
 
+- Fixes occasionally mismatching upper/lowecases of drive letters in win32 pnp check
+
+  [#7007](https://github.com/yarnpkg/yarn/pull/7007) - [**Christoph Werner**](https://github.com/codepunkt)
+
 - Fixes the error reporting for non-HTTP network errors (such as invalid certificates)
 
   [#6968](https://github.com/yarnpkg/yarn/pull/6968) - [**Chih-Hsuan Yen**](https://github.com/yan12125)
@@ -14,18 +18,18 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 - Packages won't be auto-unplugged anymore if `ignore-scripts` is set in the yarnrc file
 
-  [#6983](https://github.com/yarnpkg/yarn/pull/6983) - [**Micha Reiser**](https://github.com/MichaReiser)  
+  [#6983](https://github.com/yarnpkg/yarn/pull/6983) - [**Micha Reiser**](https://github.com/MichaReiser)
 
 ## 1.14.0
 
 - Improves PnP compatibility with Node 6
 
   [#6871](https://github.com/yarnpkg/yarn/pull/6871) - [**Robert Jackson**](https://github.com/rwjblue)
-  
+
 - Fixes PnP detection with workspaces (`installConfig` is now read at the top-level)
 
   [#6878](https://github.com/yarnpkg/yarn/pull/6878) - [**Maël Nison**](https://twitter.com/arcanis)
-  
+
 - Fixes an interaction between `yarn pack` and bundled dependencies
 
   [#6908](https://github.com/yarnpkg/yarn/pull/6908) - [**Travis Hoover**](https://twitter.com/thoov)
@@ -48,7 +52,7 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 - Fixes a bug where `os` and `platform` requirements weren't properly checked when `engines` was missing
 
-  [#6976](https://github.com/yarnpkg/yarn/pull/6976) - [**Micha Reiser**](https://github.com/MichaReiser)  
+  [#6976](https://github.com/yarnpkg/yarn/pull/6976) - [**Micha Reiser**](https://github.com/MichaReiser)
 
 ## 1.13.0
 
@@ -91,7 +95,7 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 - Properly reports the error codes when the npm registry throws 500's
 
   [#6817](https://github.com/yarnpkg/yarn/pull/6817) - [**Maël Nison**](https://twitter.com/arcanis)
-  
+
 ## 1.12.3
 
 **Important:** This release contains a cache bump. It will cause the very first install following the upgrade to take slightly more time, especially if you don't use the [Offline Mirror](https://yarnpkg.com/blog/2016/11/24/offline-mirror/) feature. After that everything will be back to normal.

src/config.js

@@ -401,8 +401,8 @@ export default class Config {
     }
 
     if (process.platform === 'win32') {
-      const cacheRootFolderDrive = path.parse(this._cacheRootFolder).root;
-      const lockfileFolderDrive = path.parse(this.lockfileFolder).root;
+      const cacheRootFolderDrive = path.parse(this._cacheRootFolder).root.toLowerCase();
+      const lockfileFolderDrive = path.parse(this.lockfileFolder).root.toLowerCase();
 
       if (cacheRootFolderDrive !== lockfileFolderDrive) {
         if (this.plugnplayEnabled) {