This fixes a regression in 1.1.1/1.1.2 where our PACKAGING docs mention
using `fetch-zig-cache.sh` but it was removed. This commit adds it back,
generating its contents from the build.zig.zon file (via zon2nix which
we use for our Nix packaging).
For packagers, there are no dependency changes: you still need Zig and
POSIX sh. For release time, Ghostty has a new dependency on `jq` but
otherwise the release process is the same. The check-zig-cache.sh script
is updated to generate the new build.zig.zon.txt file.
This brings the internal package more in line with how the nixpkgs
package is built. It also handles recursive dependencies better than the
current system.
