Add a copr project for temporary external dependencies

[LecrisUT]

• Proposing to add a separate copr project _temp_dependencies where all downstream patches needed outside of tmt control can be backported. Latest example of this is needing to backport beakerlib update that adds python3-six dependency
• Moved the packit PR copr dependencies to depend on latest (so that it is simpler to unblock on updates to fmf for example) and the _temp_dependencies for the reasons above

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Pull Request Checklist

• implement the feature

[packit-as-a-service]

We were not able to find or create Copr project packit/teemtee-tmt-4649 specified in the config with the following error:

Cannot create a new Copr project (owner=packit project=teemtee-tmt-4649 chroots=['fedora-43-x86_64', 'epel-10-x86_64', 'epel-9-x86_64', 'fedora-44-x86_64', 'fedora-rawhide-x86_64', 'fedora-42-x86_64']): repos: copr://@teemtee/_temp_dependencies does not exist..

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Unless the HTTP status code above is >= 500, please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[gemini-code-assist]

Code Review

This pull request updates copr build dependencies. The change in .packit.yaml incorrectly overrides additional_repos for the &copr-under-teemtee job, causing it to miss the latest repository. This override should be removed to allow proper inheritance.