framework/replay: Allow envvar expansion in traces YAML
This MR modifies how piglit loads traces YAML to allow for environment variable substitution before parsing YAML data.
This change improves the flexibility of the data files, allowing it to parse YAML files with environment variables.
For example, reusing the
same driver family traces specifications (freedreno, iris) with
different download-url
s for different caching proxies in CI farms.