Updates rustup to 1.20.2

Summary:
A major bug introduced by previous version is fixed:

Force-installation of toolchains because otherwise components would be marked as installed when they were not.

Test Plan: Successfully instable the stable toolchain as a regular user by running rustup toolchain install stable.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D7468