ci: use persist-credentials: false in testgen.yml

[cpu]

We were consistent about doing this in the main CI workflow, but not the testgen workflow. This is flagged when running Zizmor 0.7.0 on the repo's CI config. As mentioned in the description of this finding:

By default, using actions/checkout causes a credential to be persisted in the checked-out repo's .git/config, so that subsequent git operations can be authenticated.

Subsequent steps may accidentally publicly persist .git/config, e.g. by including it in a publicly accessible artifact via actions/upload-artifact.

However, even without this, persisting the credential in the .git/config is non-ideal unless actually needed.

We don't need it, so turn it off consistently :-)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.23%. Comparing base (8e09784) to head (ae5883c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #299   +/-   ##
=======================================
  Coverage   97.23%   97.23%           
=======================================
  Files          20       20           
  Lines        4225     4225           
=======================================
  Hits         4108     4108           
  Misses        117      117           

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