chore: hardcode v6 branch name in v6 canary publish (#6512)

Not sure if this is why the job is being skipped.

If this is the right fix, we'll want to update https://typescript-eslint.io/maintenance/releases#1-pre-release-preparation too.

.github/workflows/ci.yml

@@ -271,7 +271,7 @@ jobs:
     name: Publish the next major version code as a canary version
     runs-on: ubuntu-latest
     needs: [integration_tests, lint_with_build, lint_without_build, unit_tests]
-    if: github.ref == 'refs/heads/v${major}'
+    if: github.ref == 'refs/heads/v6'
     steps:
       - name: Checkout
         uses: actions/checkout@v3