Clean pants script which used "build"

Clean pants script which referred to "build"

[tw-172-25-146-131 pants]$ UNSET PANTS_DEV
[tw-172-25-146-131 pants]$ build-support/python/clean.sh && ./pants
Building pants.pex to /Users/tdesai/projects/pants/pants.pex...
Pants build missing a command: None

*** Pants bootstrapping failed.
[tw-172-25-146-131 pants]$

Testing Done:
yes
./pants now works.

I can add this in CI if required.

Reviewed at https://rbcommons.com/s/twitter/r/1335/
Sync of https://github.com/pantsbuild/pants/commit/47dd00954052f9259f9fa64201b31bf82cb780bb