Werk #5262: Fix bug in agent bakery (regression #5261, since 1.4.0p17)

Werk #5261 introduced a bug that made it impossible to bake new agents for some specific rulesets. This would result in error messages like the following:

"TypeError: key ('custom', 'win_certificates/lib/local/win_certificates.ps1') is not a string"

After an update, no further action is required.

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