Allow rbtools to be used from anywhere

Testing Done:
I used this to post https://rbcommons.com/s/twitter/r/1412, because I didn't have another handy way to use rbtools.

Reviewed at https://rbcommons.com/s/twitter/r/1413/

rbt

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-source build-support/python/libvirtualenv.sh
+source $(dirname $0)/build-support/python/libvirtualenv.sh
 setup_virtualenv 'rbt' 'RBTools==0.6' \
   '--allow-external RBTools --allow-unverified RBTools'
 exec rbt "$@"