Use `actions/checkout` with `persist-credentials: false`

Use `actions/checkout` with `persist-credentials: false`

Build `x86_64-apple-darwin` on `macos-15-intel`

This changes the runner in the `release.yml` workflow for the job
that builds the `x86_64-apple-darwin` target from `macos-latest`
(which currently aliases the `macos-15` runner, an Apple Silicon
system) to the recently introduced `macos-15-intel` runner. This is
to fix recent build failures for the `lean` and `max` feature jobs
with that target, which happen when the `openssl-sys` dependency
attempts to find the installed `openssl` library for the target
architecture.

The new failures can be seen in these runs:

- https://github.com/EliahKagan/gitoxide/actions/runs/17895976664/job/50882669073
- https://github.com/EliahKagan/gitoxide/actions/runs/17895976664/job/50882669084

I'm not sure why the failures happened, since they did not occur in
the `macos-15` experiments with `release.yml` don as part of GitoxideLabs#2078.

On the new `macos-15-intel` runner, see:
https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

Temporarily set `fail-fast: false` in release.yml for testing