Auto merge of #13248 - asajeffrey:etc-ci-performance-gitignore, r=shinglyu

Added etc/ci/performance downloads to .gitignore.

<!-- Please describe your changes on the following line: -->

Added the files downloaded for `etc/ci/performance` to `.gitignore`, which gets rid of lot of spurious reporting from `git status`.

r? @shinglyu

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because it's about git

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13248)
<!-- Reviewable:end -->

etc/ci/performance/.gitignore

@@ -1,5 +1,6 @@
 servo/*
 output.png
 output/*
+page_load_test/*
 
 __pycache__/