pes_events_scanner: fix processing of RpmTransactionTasks

[pirat89]

RpmTransactionTasks messages have higher priority than instructions based on PES data. Previously, if multiple such messages existed with duplicate instructions, this could lead to the crash of the actor - especially in case when an existing package has been asked to be removed several times. Ensure the occurance of each instruction is unique (list -> set).

jira: RHEL-50076

Note: just written on knees without testing. Unit tests should be updated too.

[github-actions]

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[pirat89]

/packit copr-build