Replace rust_repositories with rust_register_toolchains

[jfirebaugh]

Fixes #1224

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[UebelAndre]

Thank you so much!

Once the CLA is satisfied I'll be able to click the merge button 😄

[UebelAndre]

@jfirebaugh friendly ping. Just wanted to make sure you saw my message 😄

[jfirebaugh]

Sorry, but I don't think I can justify asking legal at Figma to review the CLA terms for such a trivial PR. If that prevents you from merging, my apologies.

[UebelAndre]

Sorry, but I don't think I can justify asking legal at Figma to review the CLA terms for such a trivial PR. If that prevents you from merging, my apologies.

No worries. I'll try to get these changes merged in a separate PR. Thanks for opening this one though!