fix mixpanel to use local vars

[braxex]

Summary

Fixes Mixpanel by using local variables. Addresses #123

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How To Test

Needs to be deployed for testing. Should provide the correct property token in deployed state.

Checklist:

• [x] My code follows the style guidelines of this project and has no linting errors
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• ~[ ] I have made corresponding changes to the documentation, including any applicable ADRs~
• [x] My changes generate no new warnings
• ~[ ] I have added tests that fail without these changes~
• [x] New and existing tests (unit, integration, accessibility) pass locally
• ~[ ] Documentation updated~
• ~[ ] If there are security concerns they are addressed or ticketed after being discussed~

[codeclimate[bot]]

Code Climate has analyzed commit 5db932c3 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 87.6% (0.0% change).

View more on Code Climate.