ci: link the nightly build

It might happen that users just copy this without updating and I'd rather have them use the current installer instead of an outdated one. This way there will be fewer updates and fewer potential issues.

Author: lazka

Diff for: web/docs/ci.md

@@ -70,7 +70,7 @@ MSYS2 yourself.
 
    ```powershell
    # Download the archive
-   (New-Object System.Net.WebClient).DownloadFile('https://github.com/msys2/msys2-installer/releases/download/2020-06-29/msys2-base-x86_64-20200629.sfx.exe', 'msys2.exe')
+   (New-Object System.Net.WebClient).DownloadFile('https://github.com/msys2/msys2-installer/releases/download/nightly-x86_64/msys2-base-x86_64-latest.sfx.exe', 'msys2.exe')
    .\msys2.exe -y -oC:\  # Extract to C:\msys64
    Remove-Item msys2.exe  # Delete the archive again
    ```