Azure: Use "pacman -Sy" to install the toolchain

We now see an error such as:
warning: database file for 'ucrt64' does not exist (use '-Sy' to download)
error: failed to prepare transaction (could not find database)

So use -Sy as suggested.

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