scdoc: Switch to fetchgit because the archive is unavailable

The Git repository is still accessible but the webpage [0] and the
download link [1] currently return "404 Not Found".

[0]: https://git.sr.ht/~sircmpwn/scdoc/
[1]: https://git.sr.ht/~sircmpwn/scdoc/snapshot/scdoc-${version}.tar.xz