ci: only deploy demo on main repo (#2015)

To prevent run [like
this](https://github.com/HigherOrderLogic/vl/actions/runs/39541535767).

.github/workflows/web-demo.yml

diff --git a/.github/workflows/web-demo.yml b/.github/workflows/web-demo.yml
index 5c1be52..0905760 100644
--- a/.github/workflows/web-demo.yml
+++ b/.github/workflows/web-demo.yml
@@ -15,6 +15,7 @@
       name: github-pages
       url: ${{ steps.deployment.outputs.page_url }}
     runs-on: ubuntu-latest
+    if: github.repository == 'linebender/vello'
     steps:
       - uses: actions/checkout@v6