bdist_pex.run() doesn't resolve cache-dir's "{pex_root"}

[weigon]

bdist_pex.run() doesn't rewrite the {pex_root} token as the "pex" cmdline frontend does which leads
to a directory named "{pex_root}/" in the root-folder of the project.

Adding the same rewrite to pex/commands/bdist_pex.run() fixes it:

if options.cache_dir:
  options.cache_dir = make_relative_to_root(options.cache_dir)

Introduced by #206

[kwlzn]

@weigon thanks for the bug report.

@digwanderlust this is related to your --pex-root change - can you take a look?