Updates rustup to 1.20.2
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
Differential Revision: https://dev.getsol.us/D7468