ingest: Use raw-string for Nextclade metadata merge rule

Preserving the line-breaks makes the command much more readable in
Snakemake output¹, which is important since I'm changing this rule right
now.

The \n previously interpreted by Python is now interpreted by `tr`,
which is preferable.

¹ <https://docs.nextstrain.org/en/latest/reference/snakemake-style-guide.html#use-triple-quoted-command-definitions>

Ported-from: <nextstrain/measles@762acdb>
Related-to: <nextstrain/measles#52>
Related-to: <#65>

ingest/rules/nextclade.smk

@@ -72,7 +72,7 @@ rule join_metadata_and_nextclade:
         metadata_id_field=config["curate"]["output_id_field"],
         nextclade_id_field=config["nextclade"]["id_field"],
     shell:
-        """
+        r"""
         export SUBSET_FIELDS=`grep -v '^#' {input.nextclade_field_map} | awk '{{print $1}}' | tr '\n' ',' | sed 's/,$//g'`
 
         csvtk -tl cut -f $SUBSET_FIELDS \