Re-add Rust overlay using current URLs

[kennystrawnmusic]

Removal of the Rust overlay is something that should never have happened to begin with. It's still maintained, but it sure as hell isn't listed.

[ajakk]

What makes you think it's still maintained? The last commits were well over a year ago, and there's a news item there that suggests you remove it.

Also, did you read the commit message dropping it (from the sole member of the Rust project)?

commit 900cee50c53be8aedd27744a3296bf4d4e0f900e
Author: Georgy Yakovlev <[email protected]>
Date:   Mon May 24 15:23:10 2021 -0700

    repositories: remove rust overlay

    rust team no longer manages the overlay and it's unmaintained.

    Signed-off-by: Georgy Yakovlev <[email protected]>

Also, rudely noting that it isn't listed is completely unnecessary.