Switch to released install.ps1

Author: dra27

.github/workflows/windows.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Download install.ps1
         run: |
           [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
-          (New-Object System.Net.WebClient).DownloadFile("https://raw.githubusercontent.com/kit-ty-kate/opam/windows-installer.debug/shell/install.ps1", ".\install.ps1")
+          (New-Object System.Net.WebClient).DownloadFile("https://raw.githubusercontent.com/ocaml/opam/master/shell/install.ps1", ".\install.ps1")
 
       - name: Restore opam cache
         id: cache-opam