devops(win): fix webkit-win64 archiving step

browser_patches/buildbots/README.md

@@ -105,6 +105,18 @@ The `core.longpaths` is needed for webkit since it has some very long layout pat
 
 Run `c:\mozilla-build\start-shell.bat` and checkout PlayWright repo to `/c/playwright`.
 
+### 7. Create a c:\WEBKIT_WIN64_LIBS\ directory with win64 dlls
+
+
+Create a new `c:\WEBKIT_WIN64_LIBS` folder and copy the following libraries from `C:\Windows\System32` into it:
+- `msvcp140.dll`
+- `vcruntime140.dll`
+- `vcruntime140_1.dll`
+
+> **NOTE**: these libraries are expected by `//browser_patches/webkit/archive.sh`. 
+
+This is necessary since mingw is a 32-bit application and cannot access the `C:\Windows\System32` folder due to [Windows FileSystem Redirector](https://docs.microsoft.com/en-us/windows/win32/winprog64/file-system-redirector?redirectedfrom=MSDN). ([StackOverflow question](https://stackoverflow.com/questions/18982551/is-mingw-caching-windows-directory-contents)) 
+
 ## Running Build Loop
 
 1. Launch `c:\mozilla-build/start-shell.bat`

browser_patches/webkit/archive.sh

@@ -83,7 +83,7 @@ createZipForWindows() {
   cp -t $tmpdir JavaScriptCore.dll MiniBrowserLib.dll WTF.dll WebKit.dll WebKit2.dll libEGL.dll libGLESv2.dll
   cp -t $tmpdir MiniBrowser.exe WebKitNetworkProcess.exe WebKitWebProcess.exe
   cd -
-  cd C:/Windows/System32
+  cd /c/WEBKIT_WIN64_LIBS
   cp -t $tmpdir msvcp140.dll vcruntime140.dll vcruntime140_1.dll
   cd -