fix: save temporary tfma output plots to temporary dir

[ghost]

Description

Fixes #29 Unit tests for the evaluation component are run with the working directory monkey-patched to a temporary directory

How has this been tested?

Unit tests still pass successfully, and files are not persisted

Equivalent PR here to show that CI checks are successful (with the exception of e2e tests for xgboost pipeline which will be fixed by https://github.com/GoogleCloudPlatform/vertex-pipelines-end-to-end-samples/pull/30)

Checklist

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have successfully run the E2E tests - [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes - [ ] I have updated any relevant documentation to reflect my changes
• [x] I have assigned a reviewer and messaged them

[ghost]

@Linchin ready to merge!