ci: rebuild tip.cuelang.org on master commits

When a new commit is submitted to master we need to trigger a rebuild of
tip.cuelang.org in order than the @master docs are updated as required.

Do that via a simple GitHub action.

Change-Id: I9d1b404756358f357c7906a4f38897c6337b4f32
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/5300
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

.github/workflows/rebuild_tip_cuelang_org.yml

@@ -0,0 +1,13 @@
+on:
+  push:
+    branches:
+      - master
+
+name: Push to tip
+jobs:
+  push:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Rebuild tip.cuelang.org
+      run: curl -X POST -d {} https://api.netlify.com/build_hooks/${{ secrets.CuelangOrgTipRebuildHook }}
+