agent_checks: Update necessary source to install package

Some alerts are about the package not being installed as it's missing that source.

Related to T3497

Test Plan

bin/octocatalog-diff says nothing.

vagrant up giverny (one of the node with that problem) and during the provisioning step, we got:

...
==> giverny: Notice: /Stage[main]/Profile::Icinga2::Objects::Agent_checks/Package[monitoring-plugins-systemd]/ensure: created

So that does fix it.

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Migrated from D6152 (view on Phabricator)