Use perl to process multi-line sha256.

[ManiaciaChao]

Regular expression sha256sums=.*$ only match from sha256sums= to EOL. Usually, more than one sources can be found in an AUR package. We usually intend to match and replace multiple lines, but sed command has its limits. Handling such case with sed command could be tricky.

However, you can simply use perl instead of sed like below:

perl -i -0pe "s/sha256sums=[\s\S][^\)]*\)/$(makepkg -g 2>/dev/null)/" PKGBUILD

P.S. I'm not going to make a PR because my fork differs a lot from yours.