fbcode_builder: minor fixup to normalize python installation paths
fbcode_builder: minor fixup to normalize python installation paths

Summary:
This just strips off a trailing '/' from the `INSTALL_DIR` property that we
set on python targets, to avoid having double-slashes in the paths that we use
during installation.  This shouldn't really have any material difference other
than cleaning up the paths that get printed during the installation phase.

Reviewed By: chadaustin

Differential Revision: D17089207

fbshipit-source-id: ab36bb76c19fa60fe037f7a5290ccfd6bdbf13b0

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly