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.
harden-runner runs with `egress-policy: block`, and the allow-list only included
`production.cloudflare.docker.com`. DockerHub serves image blobs from either its
Cloudflare or its CloudFront CDN; when a pull was routed to CloudFront
(`production.cloudfront.docker.com`) the connection was dropped, causing
`error pulling image configuration: ... connect: connection refused` and exit
125 in the xenial, installation_node, and fast (httpbin) suites. Allow both CDNs.
