ci: install bubblewrap on runners for sandbox integration tests

Phase 03 sandbox tests (Glorbo.Sandbox.Bwrap*, Integration.Sandbox*)
require bwrap on the host. GitHub's ubuntu-24.04 images don't ship
it by default. Install bubblewrap before the test step so the tests
actually run instead of hitting the BwrapHelpers.bwrap_available?()
skip branch (which also misuses {:skip, reason} in ExUnit setup).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: foobarto

.github/workflows/ci.yml

@@ -56,6 +56,12 @@ jobs:
           elixir-version: '1.18.4'
           otp-version: '28.0.2'
 
+      - name: Install bubblewrap (bwrap)
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y bubblewrap
+          bwrap --version
+
       - name: Cache mix deps + build
         uses: actions/cache@v4
         with: