Update cargo-c to 0.7.2

Summary:
Changelog:

• Use cargo 0.50.
• Pin the serde version to be compatible.

Test Plan: Rebuilt rav1e with this version.

Reviewers: Triage Team, JoshStrobl

Reviewed By: Triage Team, JoshStrobl

Subscribers: JoshStrobl

Differential Revision: https://dev.getsol.us/D10455