The unit tests fail against go v1.24

The github workflow tests secboot against go versions 1.18 and the latest
stable go version, which until recently was go v1.23. Go v1.24 was released
on 2025-02-11, and the secboot tests seem to fail against this version (and
I verified this locally).

For now, we'll temporarily remove the tests against the latest stable go
version from the workflow, and perform tests with go v1.23 instead.

I've opened this issue because I don't want us to forget that we need to
investigate why the tests are failing with go v1.24, and then we can re-enable
unit tests against the latest stable go version in the workflow once the
problem is resolved.

Start by running the secboot unit tests against Go 1.24 and inspect the GitHub workflow that selects the Go versions. Identify the cause of the failure, fix it, and verify that the workflow can again test the latest stable Go version while retaining the Go 1.18 coverage.

Written by the indexing model from the issue text.