Bump rustup from 3f9e3ba to 4f6ae53

Bumps [rustup](https://github.com/rust-lang-nursery/rustup.rs) from `3f9e3ba` to `4f6ae53`.
- [Release notes](https://github.com/rust-lang-nursery/rustup.rs/releases)
- [Commits](rust-lang/rustup@3f9e3ba...4f6ae53)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]