tests: secureboot: fix reference to unavailable kernel-module-headers

[rcooke-warwick]

2.108.6 doesn't exist for generic-amd64. The tests used to still work, because we werent acually passing the version to kernel-module-build correctly, meaning that it would default to the one in the kernel-module-build project. That was fixed in 4484c59 , and now we are passing the version correctly, we're looking for a nonexistant version. Changing version to match the default.

Change-type: patch

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[rcooke-warwick]

checking here if it fixes: https://github.com/balena-os/balena-generic/actions/runs/13072150415/job/36477111145