CI: disable GC as a workaround for segfault

.github/workflows/CI.yml

@@ -25,6 +25,8 @@ jobs:
         authToken: ${{ secrets.CACHIX_AUTH_TOKEN }}
         pushFilter: "(squashfs.img$|x86_64-linux.iso$)"
     - name: Build
+      env:
+        GC_DONT_GC: 1
       run: |
         for iso in $(nix flake show --json | nix run nixpkgs#jq -- -r keys[]); do
           nix build .#$iso