[CI] Use the latest LibCST source code in Pyre check

[jimmylai]

Summary

After we ask pyre to read site-packages, Pyre cannot understand new changes in LibCST repo..
CI installs pyre-check from pip requirements which installs the public version of libcst.
This removes the installed libcst and install from the checked out repo.

Test Plan

CI build.

[codecov-io]

Codecov Report

Merging #262 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #262   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files         214      214           
  Lines       20818    20818           
=======================================
  Hits        19546    19546           
  Misses       1272     1272

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[thatch]

Can you rename .pyre_configuration to .pyre_configuration.sample or so, now that it's ignored? (The circleci doesn't need to save and restore then, just delete the copy after running.)

[jimmylai]

@thatch Renamed .pyre_configuration as .pyre_configuration.example and updated the README accordingly.