use latest release from fmtlib instead of master
use latest release from fmtlib instead of master

Summary:
mvfst and proxygen both depend on folly, which depends on fmt, which must be built from source. the current build scripts get fmt from master, which could include a breaking change. this diff instead checks out the current latest version (6.2.1) which is known to build successfully.

related discussion in comments: https://fb.workplace.com/groups/248214662741731/permalink/601467044083156/

Reviewed By: lnicco

Differential Revision: D22213809

fbshipit-source-id: cf5efd2bf2bc694a3c246db9666088962465d4e9

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst