Since Python 3.4 `__file__` represents an absolute file path:
https://docs.python.org/3.4/whatsnew/3.4.html#other-language-changes

We only want the basename in the generated scripts.

Bug: angleproject:5516
Change-Id: Ia4009419ef627aa1ce12f9a944539c054edc3a6b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2720260Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>