Add support for DNF vars

[bootswithdefer]

Repo files can have variables, for example on Amazon Linux 2023 they use $awsdomain and $awsregion.

$ python3 dnfjson.py list
Errors during downloading metadata for repository 'asu-amazon-linux':
Error: Failed to download metadata for repo 'asu-amazon-linux': Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried
Errors during downloading metadata for repository 'amazonlinux':
  - Curl error (3): URL using bad/illegal format or missing URL for https://al2023-repos-$awsregion-de612dc2.s3.dualstack.$awsregion.$awsdomain/core/mirrors/2023.2.20231113/x86_64/mirror.list [URL rejected: Bad hostname]

$ ls /etc/dnf/vars/
awsdomain  awsregion repoguid