Auto-update zstd

GitHub release assets have checksums since early June:
https://github.blog/changelog/2025-06-03-releases-now-expose-digests-for-release-assets/

```sh
$ gh api repos/getsentry/sentry-dotnet/releases/tags/5.14.0 \
--jq '.assets[] | select(.name == "Sentry.5.14.0.nupkg") | {name: .name, digest: .digest}'
{
"digest": "sha256:362de4e2c771a40aabd82fa5683ab716fd9a7ad877ef25f969613a49e84e2bf9",
"name": "Sentry.5.14.0.nupkg"
}
```

The latest zstd v1.5.7 was released in February and only has separate manually uploaded `.sha256` files for `zstd-1.5.7.tar.{gz|zst}` but not for the `.zip` files for Windows.

```sh
$ gh api repos/facebook/zstd/releases/tags/v1.5.7 \
--jq '.assets[] | select(.name == "zstd-v1.5.7-win64.zip") | {name: .name, digest: .digest}'
{
"digest": null,
"name": "zstd-v1.5.7-win64.zip"
}
```

Starting with the next zstd release, it should be possible to query the checksum.

_Originally posted by @jpnurmi in https://github.com/getsentry/sentry-dotnet/pull/4396#discussion_r2266726432_