fix: use dnf in linux vhd tests

[liunan-ms]

What type of PR is this?
/kind fix

What this PR does / why we need it:
This PR uses dnf to download and install git instead of tdnf

Which issue(s) this PR fixes:

While running linux vhd content tests in Azure Linux VM that has preview.repo under /etc/yum.repos.d/, preview repo is known to not work with tdnf because of authentication issue with preview repo url.
dnf is the workaround to go with to work with preview repo. Generally, use dnf to install/download packages instead of tdnf.

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version
• commits are GPG signed and Github marks them as verified

Special notes for your reviewer:

Release note:

none