Regression in Vars handling

[j-mracek]

There are 3 issues covered by CI scenarios (vars.feature) that fails with DNF5

• Scenario: Variables arch supports basearch loongarch64 {}
  Substitution of multiple vars in section [dnf-ci-test-$arch-$basearch] fails

• Scenario: Using dnf with non-files in /etc/dnf/vars

• bz2091636

• Scenario: Ignoring variable files with invalid encoding

• bz2141215

[m-blaha]

The issue with invalid encoding doesn't seem to affect dnf5. The problems with loongarch64 and non-files in the 'vars' directory have been resolved by #797 and #798, respectively.