add pamac to build aur packages

[Marz de Guzman]

for install_arch.sh, I've added:
elif [ -x "$(command -v pamac)" ]; then pamac build --no-confirm polybar-git ttf-nerd-fonts-symbols
For Manjaro's package manager: pamac.

Cheers,

[Stavros Grigoriou]

Hello sir,

Would you mind making a PR with the above change?