nix: update zon2nix

- 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.

.github/workflows/test.yml

@@ -20,7 +20,6 @@ jobs:
       - build-macos
       - build-macos-matrix
       - build-windows
-      - flatpak-check-zig-cache
       - test
       - test-gtk
       - test-gtk-ng
@@ -954,33 +953,6 @@ jobs:
           build-args: |
             DISTRO_VERSION=13
 
-  flatpak-check-zig-cache:
-    if: github.repository == 'ghostty-org/ghostty'
-    runs-on: namespace-profile-ghostty-xsm
-    env:
-      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
-      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
-      - name: Setup Cache
-        uses: namespacelabs/nscloud-cache-action@305bfa7ea980a858d511af4899414a84847c7991 # v1.2.16
-        with:
-          path: |
-            /nix
-            /zig
-      - name: Setup Nix
-        uses: cachix/install-nix-action@fc6e360bedc9ee72d75e701397f0bb30dce77568 # v31.5.2
-        with:
-          nix_path: nixpkgs=channel:nixos-unstable
-      - uses: cachix/cachix-action@0fc020193b5a1fa3ac4575aa3a7d3aa6a35435ad # v16
-        with:
-          name: ghostty
-          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
-          useDaemon: false # sometimes fails on short jobs
-      - name: Check Flatpak Zig Dependencies
-        run: nix develop -c ./flatpak/build-support/check-zig-cache.sh
-
   flatpak:
     if: github.repository == 'ghostty-org/ghostty'
     name: "Flatpak"
@@ -996,7 +968,7 @@ jobs:
           - arch: aarch64
             runner: namespace-profile-ghostty-md-arm64
     runs-on: ${{ matrix.variant.runner }}
-    needs: [flatpak-check-zig-cache, test]
+    needs: test
     steps:
       - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
       - uses: flatpak/flatpak-github-actions/flatpak-builder@10a3c29f0162516f0f68006be14c92f34bd4fa6c # v6.5

.github/workflows/update-colorschemes.yml

@@ -50,8 +50,6 @@ jobs:
           if ! git diff --exit-code build.zig.zon; then
             nix develop -c ./nix/build-support/check-zig-cache.sh --update
             nix develop -c ./nix/build-support/check-zig-cache.sh
-            nix develop -c ./flatpak/build-support/check-zig-cache.sh --update
-            nix develop -c ./flatpak/build-support/check-zig-cache.sh
           fi
 
       # Verify the build still works. We choose an arbitrary build type