Use an absolute path for sourcing set_archflags.sh.

This was breaking wrapping usages from foreign repos.

Testing Done:
The error manifested like so in a Twitter internal repo, note the 1st line:
jsirois@gill ~/dev/twitter/pants_internal (jsirois/oss_pants/upgrade_scala_library_dep_injection_fix) $ PANTS_DEV=1 ./pants
/home/jsirois/.pants.dev/os_pants/build-support/pants_venv: line 5: build-support/set_archflags.sh: No such file or directory
*** Running pants in dev mode from /home/jsirois/.pants.dev/os_pants/src/python/pants/bin/pants_exe.py ***
***   with extra requirements: /home/jsirois/dev/twitter/pants_internal/3rdparty/python/requirements.txt ***
***   with extra sources /home/jsirois/dev/twitter/pants_internal/src/python ***

OSS pants bootstrapping still works.
$ git clean -fdx build-support
$ PANTS_DEV=1 ./pants
$ PANTS_DEV=1 ./pants
*** Running pants in dev mode from /home/jsirois/dev/3rdparty/jsirois-pantsbuild-pants/src/python/pants/bin/pants_exe.py ***
...

Bugs closed: 291

Reviewed at https://rbcommons.com/s/twitter/r/604/