AUR: further solve `.SRCINFO` issues

Due to the release CI job running on an Ubuntu machine, we do not have
easy access to `makepkg` to generate the `.SRCINFO` using `--printsrcinfo`
as this is a `pacman` specific utility, and instead we maintain a
template.

Historically seen, something went wrong here while the `PKGBUILD` file
became more complex and certain fields added there were not _also_
included in the `.SRCINFO` template.

This commit ensures everything is restored to working state, and
provides the proper fix for what was attempted in #2917. In addition,
checksums are now included in the file.

Signed-off-by: Hidde Beydals <hello@hidde.co>