Install `dnf copr` plugin if not present (i.e. docker containers).

[tzarc]

Description

As per title. Toolchain installation won't proceed if a package for dfu-programmer isn't found, but the install script still continues without flagging an error.

Types of Changes

• [ ] Core
• [x] Bugfix
• [ ] New feature
• [x] Enhancement/optimization
• [ ] Keyboard (addition or update)
• [ ] Keymap/layout/userspace (addition or update)
• [ ] Documentation

Checklist

• [x] My code follows the code style of this project: C, Python
• [x] I have read the PR Checklist document and have made the appropriate changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] I have tested the changes and verified that they work and don't break anything (as well as I can manage).