azure: Use msys2 20210604 installer for Windows build

MSYS2 Windows build started to fail since yesterday (Jun 21):

  checking keyring...
  checking package integrity...
  error: gcc-libs: signature from "David Macek <david.macek.0@gmail.com>" is unknown trust
  :: File /var/cache/pacman/pkg/gcc-libs-10.2.0-1-x86_64.pkg.tar.zst is corrupted (invalid or corrupted package (PGP signature)).
  error: gcc: signature from "David Macek <david.macek.0@gmail.com>" is unknown trust
  :: File /var/cache/pacman/pkg/gcc-10.2.0-1-x86_64.pkg.tar.zst is corrupted (invalid or corrupted package (PGP signature)).
  error: failed to commit transaction (invalid or corrupted package)
  Errors occurred, no packages were upgraded.

Switching to the latest installer (version 20210604) seems to fix it.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Tom Rini <trini@konsulko.com>