Add custom Arch-based act_runner image, revert workflow workaround

The runner image is now built on archlinux:base-devel with git,
curl, makepkg and a non-root builder user baked in. This removes
the need for per-workflow pacman installs and enables host mode.

ci/act-runner/Dockerfile

@@ -0,0 +1,7 @@
+FROM archlinux:base-devel
+RUN pacman -Sy --noconfirm git curl && pacman -Scc --noconfirm && useradd -m builder && echo "builder ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
+ADD https://gitea.com/gitea/act_runner/releases/download/v0.3.1/act_runner-0.3.1-linux-amd64 /usr/local/bin/act_runner
+RUN chmod +x /usr/local/bin/act_runner
+COPY --from=gitea/act_runner:latest /usr/local/bin/run.sh /usr/local/bin/run.sh
+ENV HOME=/root
+ENTRYPOINT ["/usr/local/bin/run.sh"]