1024x1024 icon not installed for Linux

[rivenirvana]

I'm wondering if leaving it out was intentional, but I doubt it since the size is supported. But in the new icon commit (247409d), a 1024x1024 size icon was introduced for use in Linux but was left unused by build.zig. I'm maintaining a COPR package for this and that's when I noticed, but instead of manually installing it in the package spec, I figured it'd be better to let the upstream build file handle this as well if it was unintentionally left out.

If so, I'd like to make a PR with these changes (main...rivenirvana:ghostty:install-icon-1024). Let me know if there's anything I missed, but I can confirm that this builds properly.

[mitchellh]

Approved #3967