Forward GITHUB_TOKEN into Linux Dagger containers for API auth

Add env: GITHUB_TOKEN to the Linux workflow step, then read it in
ci/main.go and inject it into each test container via WithSecretVariable
(for both GITHUB_TOKEN and GH_TOKEN). This prevents 403 rate-limit
errors on GitHub API calls (neovim/nvm releases) and authenticates
gh CLI for gh extension install inside the containers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/integration.yml

@@ -29,6 +29,8 @@ jobs:
           go-version: '1.25.x'
 
       - name: Run Dagger Integration Pipeline
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           go run ci/main.go --os ${{ matrix.os }}