Fix installing new zstd format packages.

See msys2/MSYS2-packages#1967

Author: winterheart

Diff for: appveyor.yml

@@ -30,8 +30,8 @@ matrix:
   fast_finish: true
 
 install:
-  # Fix installing new zstd tarballs.
-  - C:\msys64\usr\bin\bash.exe -lc "pacman -Sy --noconfirm zstd"
+  # Fix installing new zstd tarballs (https://github.com/msys2/MSYS2-packages/issues/1967).
+  - C:\msys64\usr\bin\bash.exe -lc "pacman --noconfirm -U http://repo.msys2.org/msys/x86_64/pacman-5.2.1-6-x86_64.pkg.tar.xz"
   # Update environment.
   - C:\msys64\usr\bin\bash.exe -lc "pacman -Syu --noconfirm"
   - C:\msys64\usr\bin\bash.exe -lc "pacman -Syu --noconfirm"