Reuse the established conventions rather than inventing new ones: the
AUR_SSH_PRIVATE_KEY secret name, pushing from an archlinux:base-devel
job container instead of docker-in-docker, and the existing git identity.
The AUR job only needs the PKGBUILD, so the 1.5 GB tarball never crosses
a job boundary - it goes straight to GitHub Releases from the build job.
Also correct the Gitea job-timeout setting name to ENDLESS_TASK_TIMEOUT.