Ensure order of `vars:` and `vars_files:` is consistent with `--var` and `--load-vars-from` flags

Summary

Currently, the order appears to be 'vars > file1 > file2`:

https://github.com/concourse/concourse/blob/fab3de1722a2ce998d3710bd066453594f24ec57/atc/exec/set_pipeline_step.go#L179-L196

We should change this to match the documented behavior of fly set-pipeline -l -v, i.e. vars: takes precedence over files, and later-listed files take precedence over earlier-listed files.

This is technically a breaking change but the set_pipeline step is already marked experimental, so that's not a big deal.

Context

• Prior discussion: #5446