Fix bdist_pex interpreter cache directory (#286)

Running bdist_pex would result in the creation of a directory literally named '{pex_root}'.

This fixes it by following behavior similar to that found in pex.bin.pex:main

pex/commands/bdist_pex.py

@@ -77,6 +77,7 @@ def run(self):
     # Update cache_dir with pex_root in case this is being called directly.
     if options.cache_dir:
       options.cache_dir = make_relative_to_root(options.cache_dir)
+    options.interpreter_cache_dir = make_relative_to_root(options.interpreter_cache_dir)
 
     if options.entry_point or options.script:
       die('Must not specify entry_point or script to --pex-args')