mkinitcpio: add zstd dependency

[tranzystorekk]

Testing the changes

• I tested the changes in this PR: briefly

cc @ahesford

[classabbyamp]

new version is coming pretty soon, is this super urgent?

[tranzystorekk]

@classabbyamp not really, i can close, just gotta remember to include these changes

[ahesford]

zstd is optional because not all kernels we ship use zstd-compressed modules and, even for those that do, the errors printed when mkinitcpio tries to decompress the modules by default (which can be disabled) don't prevent the original, compressed modules from being included.

Nevertheless, if the errors are alarming to users and the solution (installing zstd) isn't obvious, we can add the dependency.

[ahesford]

I pushed the update to v37 and included the homepage change. Using the Gitlab archive in distfiles breaks the version number recorded in /usr/bin/mkinitcpio, so I reverted to sources.archlinux.org for a tarball that contains proper version information.

I'm still on the fence about zstd and didn't include that in the depends with the latest update.

[ahesford]

It turns out that Arch considers zstd a hard requirement even as they use the optdepends mechanism for other compressors: https://gitlab.archlinux.org/archlinux/packaging/packages/mkinitcpio/-/blob/main/PKGBUILD

That makes a convincing enough argument that we should just pull it in, so we can merge this if you rebase.