Bump rand_core from 0.6.3 to 0.6.4 (#2541)

Bumps [rand_core](https://github.com/rust-random/rand) from 0.6.3 to 0.6.4.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@rand_core-0.6.3...0.6.4)

---
updated-dependencies:
- dependency-name: rand_core
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>