Verify checksums for release files downloaded from github

See https://github.blog/changelog/2025-06-03-releases-now-expose-digests-for-release-assets/

The digests are only available for recent releases (e.g. [this one from four days ago](https://github.com/cargo-bins/cargo-binstall/releases/tag/v1.12.6) has them, but not [this one from three weeks ago](https://github.com/cargo-bins/cargo-binstall/releases/tag/v1.12.5)), not all of them retroactively.