Fix appveyor builds with pacman update workaround

[bvogelzang]

Appveyor builds were failing when updating packages:

:: Proceed with installation? [Y/n] 
:: Retrieving packages...
downloading bash-5.1.004-1-x86_64.pkg.tar.zst...
downloading filesystem-2021.01-1-x86_64.pkg.tar.zst...
downloading mintty-1~3.4.4-1-x86_64.pkg.tar.zst...
downloading msys2-runtime-3.1.7-4-x86_64.pkg.tar.xz...
downloading msys2-runtime-devel-3.1.7-4-x86_64.pkg.tar.xz...
downloading pacman-mirrors-20201208-1-any.pkg.tar.xz...
downloading zstd-1.4.8-1-x86_64.pkg.tar.zst...
downloading pacman-5.2.2-9-x86_64.pkg.tar.zst...
checking keyring...
checking package integrity...
loading package files...
error: could not open file /var/cache/pacman/pkg/bash-5.1.004-1-x86_64.pkg.tar.zst: Child process exited with status 127
error: could not open file /var/cache/pacman/pkg/filesystem-2021.01-1-x86_64.pkg.tar.zst: Child process exited with status 127
error: could not open file /var/cache/pacman/pkg/mintty-1~3.4.4-1-x86_64.pkg.tar.zst: Child process exited with status 127
error: could not open file /var/cache/pacman/pkg/zstd-1.4.8-1-x86_64.pkg.tar.zst: Child process exited with status 127
error: could not open file /var/cache/pacman/pkg/pacman-5.2.2-9-x86_64.pkg.tar.zst: Child process exited with status 127
error: failed to commit transaction (cannot open package file)
Errors occurred, no packages were upgraded.

This issue https://github.com/msys2/MSYS2-packages/issues/2300 mentions the same problem and suggested a workaround by updating zstd and pacman first, before doing the full package update. Which seems to have done the trick.

[aharpervc]

Awesome, thanks!