Remove relative directory reference in mojo\golden\generate.py

The relative directory reference prevents presubmit changes from
executing if the repository is located on a separate drive from the
output directory.

The relative directory reference was only used for error reporting.
Instead of using the relative directory reference, the output dir
can be used directly.

Bug: 393431145
Change-Id: Ibeec204040bef6a157bbe3a99bd49f59bdd6e115
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6220828
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Fred Shih <ffred@chromium.org>
Commit-Queue: Emily Andrews <emiled@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1414307}