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.
Jordan Harband
77fca77ddd