[actions] fall back to the canonical nvmrc submodule when a fork lacks its own

Upstream's `.gitmodules` uses a relative submodule url (`../nvmrc.git`),
which resolves against the superproject's origin:
on a fork without its own `nvmrc` fork that is `<owner>/nvmrc`,
which 404s and fails `actions/checkout` with `submodules: true`.
Keep `submodules: true` (so a fork's own `nvmrc` is used when present),
but mark checkout `continue-on-error` and, only when it failed,
re-point the submodule at `nvm-sh/nvmrc` and update.

.github/workflows/tests-xenial.yml

@@ -42,8 +42,17 @@ jobs:
             registry-1.docker.io:443
             auth.docker.io:443
       - uses: actions/checkout@v6
+        id: checkout
+        continue-on-error: true
         with:
           submodules: true
+      - name: 'nvmrc submodule fallback (forks without their own nvmrc)'
+        if: steps.checkout.outcome == 'failure'
+        shell: bash
+        run: |
+          git submodule set-url test/fixtures/nvmrc https://github.com/nvm-sh/nvmrc.git
+          git submodule sync --recursive
+          git submodule update --init --recursive
       - uses: ljharb/actions/node/install@main
         name: 'npm install && version checks'
         with: