Fix security-related issues in github actions flagged by zizmor

[peytondmurray]

Issue number of the reported bug or feature request:
Closes #18.

Describe your changes
This PR resolves security-related issues with the github actions flagged by zizmor, and adds zizmor as a github action to ensure we don't introduce new security vulnerabilities in the future.

I also removed the PYTHON=... in the make lint call. There is no matrix.python-version variable, so this wasn't doing anything.

Testing performed
Testing will be done as part of CI.

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[sarahmonod]

LGTM although I suspect @godlygeek will want to rewrite the commit message. Also I wonder if we should add Dependabot here, given that the upload and download actions seem out of date to me.