check whether dnf command exists before aliasing

[shinenelson]

Description

Enclose the aliases for dnf within the _command_exists helper function.

Motivation and Context

We do not need to add aliases for package managers if the command does not exist.

This was done for other files that were already checking for commands via command -v in #1938.

How Has This Been Tested?

Tested by syncing profiles between Debian, Fedora and macos

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[seefood]

yup