Add a script to update build-support/pants_requirements.txt.
Ran that script and it generated a slightly different output (no more
system dependencies).

Also fix a bug in the pants script that causes us to bootstrap pants
on every run (missing $PANTS_VERSION).

Testing Done:
Ran the update requirements script and ran pants multiple times to
confirm it doesn't bootstrap.

Reviewed at https://reviews.apache.org/r/27097/