[Nix] Re-package arcanist as just the php scripts

Summary:
Nixpkgs arcanist pacakge includes "phage" shell which
is a fairly large c++ and php code base. However,
we only really care about using the arc command, so we should reduce
the scope of the arcanist package to be what we actually use.

This avoids situations such as https://phab.comm.dev/D4483#131136
where the build for phage (and it's requirements), aren't thread safe. They
use bison and flex, and the generated files aren't delcared in a way in which
make will be aware that generation of those files need to be ordered before
usage of them.

Test Plan:

# this can be used from any branch
nix develop github:commE2E/comm?ref=update-nix-flakes
# use any arc command, 8.1 would fail on `arc diff`

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4662