ci: add test-lib-vt job

Add a new CI job that runs `zig build test-lib-vt` to test the
lib-vt build step. The job mirrors the existing test job structure
with the same nix/cachix setup and skip conditions. It is also
added to the required checks list.

.github/workflows/test.yml

@@ -105,6 +105,7 @@ jobs:
       - test-sentry-linux
       - test-i18n
       - test-fuzz-libghostty
+      - test-lib-vt
       - test-macos
       - pinact
       - prettier
@@ -911,6 +912,36 @@ jobs:
       - name: Test System Build
         run: nix develop -c zig build --system ${ZIG_GLOBAL_CACHE_DIR}/p
 
+  test-lib-vt:
+    if: github.repository == 'ghostty-org/ghostty' && needs.skip.outputs.skip != 'true'
+    needs: skip
+    runs-on: namespace-profile-ghostty-md
+    env:
+      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@a90bb5d4b27522ce881c6e98eebd7d7e6d1653f9 # v1.4.2
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@1ca7d21a94afc7c957383a2d217460d980de4934 # v31.10.1
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@1eb2ef646ac0255473d23a5907ad7b04ce94065c # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Test
+        run: nix develop -c zig build test-lib-vt
+
   test-gtk:
     strategy:
       fail-fast: false