refactor: read minimizerReferences from buildFiles instead of files

Current Nextclade versions reject array values in the `files` section of `pathogen.json` with `invalid type: sequence, expected a string`. The `DatasetFiles` struct catches unknown keys with `rest_files: BTreeMap<String, String>` (https://github.com/nextstrain/nextclade/blob/f7db57f31/packages/nextclade/src/io/dataset.rs#L567-L569), which only accepts string values. An array like `minimizerReferences` fails deserialization before reaching the `other: serde_json::Value` catch-all.

Move the lookup to `.buildFiles`, a top-level key absorbed by `VirusProperties`'s `other: serde_json::Value` (https://github.com/nextstrain/nextclade/blob/1400012f7/packages/nextclade/src/analyze/virus_properties.rs#L137-L138), which accepts any JSON type. This makes datasets with multiple minimizer reference files loadable by both old and new Nextclade versions.

Author: ivan-aksamentov

scripts/rebuild

@@ -177,11 +177,11 @@ def get_minimizer_refs(pathogen_json, dataset_dir):
   """
   Get reference sequences for minimizer index.
 
-  If minimizerReferences is specified (array of file paths), returns sequences
-  from all files combined. Otherwise, returns the main reference sequence.
+  If buildFiles.minimizerReferences is specified (array of file paths), returns
+  sequences from all files combined. Otherwise, returns the main reference sequence.
   """
-  files = dict_get(pathogen_json, ["files"]) or {}
-  minimizer_refs_filenames = dict_get(files, ["minimizerReferences"]) or []
+  build_files = dict_get(pathogen_json, ["buildFiles"]) or {}
+  minimizer_refs_filenames = dict_get(build_files, ["minimizerReferences"]) or []
 
   if len(minimizer_refs_filenames) > 0:
     all_refs = []