- Builds with Zig 0.15 now (but still works just fine with Zig
  0.14 projects).
- Fixes a double-free if nix-prefetch-git can't be found or errors out
- Adds support for generating Flatpak package metadata natively.

This adds two explicit `zig build` steps: `run-valgrind` and
`test-valgrind` to run the Ghostty exe or tests under Valgrind,
respectively.

This simplifies the manual Valgrind calls in a few ways:

1. It automatically sets the CPU to baseline, which is a frequent and
   requirement for Valgrind on newer CPUs, and generally safe.

2. It sets up the rather complicated set of flags to call Valgrind with,
   importantly setting up our suppressions.

3. It enables pairing it with the typical and comfortable workflow of
   specifying extra args (with `--`) or flags like `-Dtest-filter` for
   tests.

Works around https://git.lix.systems/lix-project/lix/issues/520

Setting `zig/flake-compat` to follow `""` (the current flake, ghostty)
is incorrect and introduces an unnecessary dependency cycle.