Install

Remove deprecated signatures

    sudo rm -f /var/lib/pacman/sync/mesa-nonfree.*.sig

Locally trust CI key

    sudo pacman-key --recv-keys B728DB23B92CB01B && \
        sudo pacman-key --lsign-key B728DB23B92CB01B

Switch to Manjaro unstable branch and

    sudo pacman-mirrors -aS unstable && \
        sudo pacman -Syu

Download repo configuration

    sudo sh -c "curl -s https://nonfree.eu/unstable/ > /etc/pacman.d/mesa-nonfree.pre.repo.conf"

Inject repo configuration in pacman.conf

The config must precede the core repo to work

    sudo sed -i '/^\[core\]/i \Include = /etc/pacman.d/mesa-nonfree.pre.repo.conf\n' /etc/pacman.conf

Remove

Remove repo and return to stock Manjaro

    PKGS="$(pacman -Sl mesa-nonfree | grep 'installed' | awk '{print $2}')" && \
        sudo sed -i '/mesa-nonfree/d' /etc/pacman.conf && \
            sudo pacman -Sy $(echo ${PKGS//$'\n'/ })